On the Macintosh these files are really two file 'forks'.
When they are transferred to a PC these two forks may be flattened (preserving all data and parameters) or only the data fork (containing the raw sound data) may be copied.
If only the data fork has been copied, then all information about sample rate and data format has been lost when it was moved to the PC and this program will simply assume that it is 16-bit mono data at 44100 Hz.
If the data should in actual fact be of another format or another sample rate, then you need to manually go into the waveform properties dialog and enter the correct sample rate in the
General page, and the data format in the
Source page, then hit