First, do you have the latest Excel Add-in installed?
Is the add-in showing within the Installed programs of the client OS?
Does the user have admin rights to install the add-in ((Note on the latest ALM 11 and ALM 11.5x add-ins admin capabilities are not needed.)?
Was the ALM Client registration run after the add-in installed?

If all of these things have been done, then you may need to have the MS Office registered in the client. This can be accomplished in the following 2 ways: repair/ reinstallation of MS Office or manually registering the file.

To perform these, follow the following recommendations:

Here are the options/ commands to register the client files:

Repair or reinstall Microsoft Office. By default the mscomctl.ocx file is registered when Microsoft Office is installed.

Manually register the mscomctl.ocx file:

For Windows 7 64bit OS, run the following command in the cmd window with administrator privileges:
Regsvr32 c:windowssyswow64mscomctl.ocx

For Windows 7 32bit OS, run the following command in the cmd window with administrator privileges:
Regsvr32 c:windowssystem32mscomctl.ocx

For Windows XP OS, run the following command in the cmd window with administrator privileges:
Regsvr32 c:winntsystem32mscomctl.ocx

I hope this helps,
Dan