I believe you need to export or save the 2007 version as the older xls version then you should be able to import.
Thank you for your post.
The reply from "HugoLidia" is correct (Thank you!). FileMaker Pro 9 will only import .xls files. The .xlsx file import was added to FileMaker Pro 10.
When in Excel 7, save the file as a .xls file. Then, you can import it into FileMaker Pro 9.