Thank you for your post.
Converting from an earlier version of FileMaker can be very simple or complex depending on the number and complexity of your files. We have a document that we strongly recommend you review before converting files. It is Answer ID 750 in the FileMaker knowledge base, and you can find it at this link:
And this link addresses the conversion methods: http://help.filemaker.com/app/answers/detail/a_id/5366
You do need to convert the solution created in FileMaker Pro 6. What method did you use to open the older files with FileMaker Pro 11? Did any messages display? Additional details would help me assist you further.