Several causes are possible:

1. Please check if the Outlook® add-in is deactivated. If this is the case, please activate it again. Outlook® 2000-2003: Help ->  About Microsoft® Office Outlook® -> Disabled Items; Outlook® 2007: Tools -> Trust Center -> Add-ins -> Manage-> Disabled Items -> Go...
   
   
2. Try a reinstallation: start -> Programs -> ESM-Tools -> Change -> activate Install again -> Execute.
   
   
3. May be a program dll was not registred by the setup. In such a case please register manually. Please close all Office programs before! To register admin rights are required which you can get like this:
   Start -> (All) Programs-> Accessories -> Right click on Command Prompt -> Run as... and/or Run as administrator (in Windows Vista®) and log on as administrator. Please note that this command will only work if the admin account is protected by a password.
   If not already set, change to the directory System32: cd c:\windows\system32 -> ENTER
   Now enter the following command and pay attention to the quotation marks:
   Windows® XP: regsvr32 "c:\programme\esm-tools\%program name%\%program name%.dll" -> ENTER
   Windows Vista® 32-bit: regsvr32 "c:\program files\esm-tools\%program name%\%program name%.dll" -> ENTER
   Windows Vista® 64-bit: regsvr32 "c:\program files (x86)\esm-tools\%program name%\%program name%.dll" -> ENTER 
   Please replace %program name% with the program name e. g. FormatNumbers (upper/lower case is not important)
   
   
4. If Outlook® starts minimized the Add-Ins will not be loaded (bug in Outlook®). A workarround was published on June, 15th 2008. Many thanks to Michael Bauer who provided code for this not easy workarround.
    
5. Outlook® sometimes remain in the main memory after the shut-down. It migth be the case that the add-ins can not be reloaded if you want to start Outlook® again. Therefore please check after the shut-down if the process Outlook.exe is still running in the task manager. If this is the case and the process doesn't close by itself after about 20 seconds, close the process by hand or log-in again (rebooting is not necessary).
   
   
6. Possibly there is a problem with the FM20ENU.DLL. See also FAQ Outlook® crashes if I click on the program icon.
   
   
7. Your antivirus software might be the problem. Especially Norton-Internetsecurity is known for problems in combination with Outlook®. A mere deactivation of the software right before installation is often not sufficient. To make sure that the failure is not caused by the antivirus software, it`s recommend to uninstall the software temporarily or install it on another (virtual) machine (if available). Don't be scared to uninstall the software. Just unplug the connection to your network interface card/modem to cut the connection to the world wide web. If you've already a virus on board, you should install an alternative product.
    
8. There could also be problems in relation to registry cleaners or tuning tools. Please check if you find the entry
   
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\Addins\%NAMEOFPROGAM%.ComAddIn\LoadBehavior
   
   in the registry. Set the value on 3. If not please do like described in step 3 above.
   
   
9. There are two different ways to install the software (if you use the userdefined setup): Install only for me and Install for all users. If the program is installed by an admin for a user with restricted access (with Run as...), the way Install for all users has to be chosen. With the option Install only for me the program would only work if the admin is logged in.
   
   Notice: If the option Install for all users was chosen, the add-in is available and active, but not visible under Tools -> Options -> Other -> Advanced Options... -> Com-Add-Ins...
   
   To check if the program is installed correctly please go to Help, About Microsoft® Office Outlook®, System Info. After some seconds the window System Information should open. In the category Software Environment, Loaded Modules there should be visible e.g. FormatNumbers on the right part of the window.

If all these advices didn't help, please check your system for updates. To get the latest downloads please visit Windows®- or Office-update page. Servicepacks are available not only for the operation system but also for MS Office.

If all components are up to date, please try the function Detect and Repair... right under Help in Outlook®. Be careful and do not check the boxes because this leads to the extinction of all E-mail accounts and MS Office settings.

If you're still in trouble, I offer you a remote maintenance for free to solve the problem directly on your PC.