Or--Audacity runs rather nicely on mac; as does VLC--good tools.
You seem to have a knack for making up some good riffs, now the real skill in composing is not composing, rather, arranging. In the first track you posted, there were some good compositional ideas--motifs. Now, you can develop these motifs to flesh out your song. For example, the synth lead at the beginning was pretty cool, but it never came back!
You made variations on the primary licks (that start at ~00:31), which is great. But, you can do even more. All Bach did when he composed some of his best music was think up a nice sequence of notes, not even long enough to be called a melody, and went nuts with them. You can play them up a step or two (or more if you'd like), play them backwards, upside down (invert the intervals between notes), or with different instruments.
Here's a really good visual representation of an analysis of one of Bach's most popular songs, the two-part Invention No. 1 in C Major:
http://jan.ucc.nau.edu/tas3-cgi-bin/nph-canonx.pl?track1.vcd&invanalysis.html