you need a chinch to pin (?) cable...put the chinch into your portastudio (should be marked red and white and say line out or something like that).the other end should be in your computer (line in, mostly blue). then you need a recording software like wavelap etc. there are free ones like audacity which get the job done.they can record your mixed songs of your portastudio. you need to check that you have set up "line in" as your recording source in your computer.check the levels of your recording software while recording, peaks should be at -1db or lower.you can get up to 0 db but higher will result in distortion/clipping and sounds awful ^^
I'm not english and bad when it comes to explaining...hope it helped anyway
btw: the mp3 s posted here are made with other portas like the 244,242 (II,III),464,644 etc. which all have a real mixer section with EQ,effect inputs,better preamps and dbx noise reduction (eliminates hiss of the tape)