Beat matching takes lots of skill, and practice. For starters you can always count the # of beats in 15 seconds then x4=BPM When you get more skilled you can match BPM by simply looping a short 4-8 beat in your head. Use a pair of good headphones, and listen to the song that you want to mix into with the headphones in one ear, while listening to whats playing with the other ear (your only using one side of the headphone, leave your other ear uncovered to listen to the speakers/monitor). Then what you do is figure out if the song u want to mix into is slower or faster... change the pitch until it sounds about the same speed as the song playing. Ok so you got them at pretty close BPM now they have to be matched, use the pitch bend to temporarily speed up the track(about to be played) until they are matched. It takes a fair bit of practice before you get good. It also helps if you get a good instructional DVD so you can visually see how it's done.
Here is an example:
2 tracks, both at different BPM:
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2 Tracks at the same BPM but not matched(ignore the green smileys I had to use smileys so it would work):
Now matched up:
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