Here are details about audio files (samples) that can be imported into the TD-50.
Format - .wav
Bit rate - 16, 24 bit
Sampling Frequency - 44.1kHz
Length of each file: Maximum 180 seconds
If you're sure you have a .wav file that matches this criteria and it still won't import into the TD-50, then the file may have embedded metadata, or "tags," which will prevent it from importing. This can happen with files that come from DAWs ("Digital Audio Workstations"), other audio programs, or sample libraries.
You can strip this extra information from the file by converting it to Apple Lossless or FLAC format. After removing the information, the files can be imported TD-50. You can use various free programs (iTunes etc.) to perform this conversion.