on fruity on the play list (F9) theres two parts, the top is for sequences and patterns, the bottom is for automation and audio
if u have a new file u can simply click on the lower part of the playlist and a browse section come up and u can select which audio file u want in the playlist as long as it is compatible with fruity, u can now use mp3 files on Fruity 6 : p
if that doesnt work then open up the step sequencer (F6) and right click on one of the channels....
Insert ---> Audio Clip
Straight away the channel settings appear and u can click on the folder icon to browse for your file
once loaded the file will appear as a wave form that is visable, u can click and drag this into the playlist
hope i helped dude