From version 3, Photoshop are binary compatible and can be freely moved between Macintosh and PC. Under version 2.5, you had to save the file on both platforms as TIFF files or EPS files.
Transferring DCS files between Macintosh and PC can be difficult, because the main file (the masterfile) contains pointers to the color separations. Please see our FAQ for a description of the problem (with a solution, of course). See also the paragraph on the signature of those files and how to get correct EPSF signatures even for the color seps.
Photoshop can't rasterize the ESP files produced by Freehand. In this case, the program uses the low-resolution preview picture. Results are of course bad.
The solution is to export the file as an Illustrator 3 format. The file is then properly rasterized in Photoshop.