XLC checks that XLC.dll and XLC.xll are present and installed as Excel addins. If they are not it will try to install them. It sound like something is going wrong with this process. Can you try the following for me
1) What version of Excel are you using and what version of Windows?
2) Do you have user privilege to install software?
3) Have you successfully installed other Excel Addins?
4) Re-install XLC
5) Do you have a directory called C://program files/XLC and what files are contained there?
6) Do you get a "#NAME?" error when you insert the formula "=OVD(1)/UND(2)"?

Lets see where this takes us.
Cheers
John