You might see an "invalid certificate" error when installing Office for Mac 2011 or Lync for Mac 2011. This occurs because a few of our install and update packages are signed with an older certificate. You'll simply need to download a package for a newer version, which uses an updated certificate, to use for your installation. This error does not impact your license or subscription.
Note NOV2020: If you just copy/paste the last file "/Library/Preferences/com.microsoft.office.licensing.plist" as instructed above, it should work as well. For transparency, this was tested on an old mbp with El Capitan and incoming connections blocked with Firewall (unsure if Firewall would affect outcome).
If this doesn't work for you (ie. you probably don't own a legitimate copy of the program) I'm afraid you're out of luck, I believe the licensing information may be stored in the com.microsoft.office.licensing.plist file which is located in ~/library/preferences but it's encrypted so... good luck with that.
Download an IQY file. Be sure to select "download file" instead of "open with Excel". Save the file in "My Documents/My Data Sources" in Windows or "/Applications/Microsoft Office 2011/Office/Queries" in MacOS. 2b1af7f3a8