Unfortunately, the method we use to automatically detect chords doesn't directly translate to working out bass lines. A bass-line detector is quite a popular request though so we plan to do some investigating in the future to see if we can make it happen!
A lot of Riffstation’s features can be quite useful for bass players as is though. You can use the loop, isolate and slow down tools as they are. The chord strip view in Riffstation also functions as a pretty decent bass line guide.
Bass doesn't generally isolate as cleanly as guitar but the results will vary depending on how the song was recorded and produced. I've attached a screenshot showing a typical bass setting. You can select M or S depending on whether you want to solo or mute the part you have selected. It sometimes helps to shift the isolated bass part up an octave using the pitch tool to make it easier to work out the notes.