If you have a TIFF file that Claris CAD will not display as openable in the Open File or Place File dialogs, the file may have an internal format that Claris CAD does not recognize. This is especially likely if the TIFF file came from a non-Mac platform. Claris CAD determines whether it can read the file by the files internal structure rather than by type or creator.
You may be able to open the file in Photoshop and save it out using the LZW compression routine. To do this, select Save As... from the File menu, then select TIFF as the file format. When you hit Save, a dialog comes up with the TIFF options, Click the LZW compression check box and hit OK. Claris CAD should be able to read the file.