First, you use FileMaker 11 to convert your 5.5 file to .fp7 format, then you use Filemaker 12 to convert to .fmp12 format. (.fp7 format is the file format used by versions 7 - 11)
Here's a download link for a 30 day free trial copy of Filemaker 11 you can use for the first step of the conversion:
I also recommend that you read these two Knowledge Base articles on converting your files:
Converting to .fp7 format: http://help.filemaker.com/app/answers/detail/a_id/750/kw/converting
Converting to .fmp12 format: http://help.filemaker.com/app/answers/detail/a_id/10571/kw/converting
Be prepared to invest some time into testing and updating the converted files as there is a good chance that your converted files may not function perfectly. The KB articles cover many, but not all of the possible issues. Feel free to post back here with any issues you encounter that don't seem to have been covered by the KB articles.
PS. You have posted this to Report an Issue--the part of the forum that is intended for reporting possible software bugs. To get help using FileMaker Pro, your post will be seen by more people if you post it to the FM Pro Forum. (See tabs at top of this screen.)
But I see that ypu have sent me another reply that answers my question perfectly. Thank you so much!