We have not had many installation issues reported in recently so thanks for letting me know your bug. Your problems sound strange so let's do a few checks to see what's happening.

1) Start Excel. Go to Tools>Add-Ins... This displays the Add-ins form and check that xlc is shown as installed. If OK then proceed.

2)Is the XLC toolbar visible? Check Excel MainMenu>View>Toolbars> and XLC Tools should show with a check mark next to it. If OK then proceed.

2) Check that XLC function are working. Type "=OVD(3)/und(6)" into a cell. If the cell returns 0.5 then XLC functions are working OK. If OK the proceed - if not install XLC.xll manually by following these instructions. From the add-ins form select the browse button and navigate to your XLC installation directory (normally this is Crogram Files>XLC unless you installed it elsewhere). Select the xlc.xll and press 'OK'. This has manually installed xlc.xll

Let me know how you get on.