Also, you're clearly not using option explicit at the top of your modules. Put that at the top of each code module (there is a setting to do this automatically) under tools/options.

Option Explicit will help you more than any forum. That will help you see other errors in your code... But you need to step through. Alternatively, if you think you're having trouble because of porting to excel, copy the mail merge code back into word, and try it out. Since it has a parameter, you'll either need to create a parameter-less routine that just contains one line:
MailMergeToExcel ActiveDocument
Or you could type that in the immediate window to test (all in Word VBA instead if Excel VBA.