SonicAlbert is absolutely right. The compressor is one of the hardest tools to learn, but once you do learn it you will regret buying that little box with presets. You are on the right track if you cannot hear it working. The compressor should be seen (as in watching the meters to see what its doing) and not heard (as in the sound should fool your ear into believing it has not changed). If you can hear it working it's too much compression.
The easiest way I've found to actually toy with a compressor so you can hear what it does is to run a kick drum thru it. A drum machine kick works great for this because it will repeat over and over while you turn the knobs on the compressor. But a bass will work well too.
Set the threshold to 0, ratio to 1:1, attack to 50ms, release to 500ms, and gain to 0. Play a repeating kick drum sound thru it, one that hits really hard and has a lot of punchy attack to it and the input meters go over 0 quite often. Then turn the ratio knob up slowly and watch the gain reduction meters. The higher the ratio knob goes the more gain reduction meter you'll see lighting up. Set the ratio high, like 10:1, then manually tap the kick drum and watch how the gain reduction meter lights up more the harder you hit the kick. The harder you hit it, the more it tries to go over 0, and the more the compressor knocks it back down in an effort to keep it from going over 0.
Hit the kick really hard and turn the attack knob to 1ms and notice how the crispy attack of the kick disappears. The attack is soo fast that the compressor clamps down on the kick before the sound can even get fully developed, making it sound dull and lifeless. If you go the other way and set the attack not fast enough, the kick will complete it's sound "curve" before the compressor ahs a chance to kick in and keep it from going over 0.
A basic compressor manual
I know a lot of people knock the Alesis 3630, but the manual is straight forward and easy to understand, it is a good primer for what a compressor is supposed to do. Dont give up on it, learning the compressor is a huge step toward making your own great demo mixes. - SC