Can't transfer files from old Mac to new one

Hi everybody, I have 2 Mac OS X laptops- when I just got 2nd one it was no problem moving I-Tunes into 2nd one using Migration Assistant. But later after I download some tracks from CDs or from Flash into one of my Macs I thought I could use Migration Assistant to transfer them to 2nd one- but what happened- it ate some space but it didn't transfer those tracks. so I decided to clean my 2nd Mac as much as I could and use migration Assistant again - things got even worse. Now It occupied even more of the space, leaving 2 GB available (my first Mac has 53 Gb avail), but playlists are not loaded, and most of tracks won't play - when I click on it the sign says "The original couldn't be located". So I need to start all over but I'm confused now- What should I delete completely on my 2nd Mac to free the space and how to transfer everything including I-Tunes from my 1st one? Thanks a lot