This is usually due to your Word security settings. If you set the security settings high enough, and don't specifically accept our add-in when the security warning dialog comes up in Word, it will not be allowed to run.
To see if this is the case, run Word and go to the security settings (the exact location depends on the version of Word, for Office 2007 go to to Tools\Macro\Security. If the setting is currently on High, try lowering it to Medium or Low and rerunning Word. If the LEC add-in appears, it means that it is not considered a "trusted source" (you can verify that by going back to Tools\Macro\Security and clicking on the "Trusted Sources" tab). After this, if you reset the security to High, it will either keep working or bring up a dialog box asking whether you should trust software from "Language Engineering Company," to which you should answer "yes/enable."
If you are using Outlook as your email editor, you may need to open Word first, then start Outlook. If you start Outlook before starting Word, then all add-ins in Word, including ours, will not run.
You may also open your Word or Excel program, then select Add-Ins or Template & Add-Ins from the Tools menu, and make sure the "LEC Translate.dot" file is checked.