You should be able to serach the internet and find instructions on how to add freon. That's actually easy enough to do. I've done it planty of times to automotive A/C and have watched other people do it on home A/C once ot twice. The difference being that on a car A/C I was uaully just adding increments in terms of the number of cans until I got cold air out of the vents. With a house A/C, you could concieveably do the same thing, however, with a house A/C you really need to know information about the compressor, and figure in things like the outside temperature to get the right amount. The difference being that for a house A/C you want the optimal amount because too little and your A/C is inefficient and too much causes problems as well. I'm not saying that you can't do it yourself, I'm saying that if you really want it done right, you need to do a lot of reasearch first.
And as for all the people saying fix the leak, they have a point too, but I'd use it as an excuse to upgrade if your old unit is less than 10 SEER and you use it a lot. You didn't say where you live. I live in Baltimore, and if I had a problematic unit that was less than 10 SEER, I'd trash it. If you live in Canada or someplace else where you don't use much A/C, then maybe even 8 SEER would still be OK.