you should be able to import tiff-files from the Import-panel. I am doing it all the time, for example:
What kind of tiff-files do you have? 16 bit, 8 bit, what is the file-extension? Are the tiff directly from your camera or did you generate them using a program?
What are your import setiings? Can they be excluding duplicates, special file types?
And what is your Aperture version and your MacOS X version?
If you remove the "Raw+JPEG Pairs" section, you should get all your importable files listed. I find it easier to add/remove the "Raw+JPEG Pairs" section as needed, than to change the setting to something that isn't actually what I want even if it gives me the result I need.