Bicycle Mechanics - Help removing freewheel cassette

So, i'm building a singlespeed bike out of a nice bike and a not so nice bike-, and i'm trying to get the gears off the super old, and well, not so awesome rear wheel so i can put a bmx free wheel. I got the little tool dealie; and tried to pull them off, but the thinggie seems to be totally seized. any suggestions on how to get it off? is there some secret bolt that i don't know about?

First of all, a freewheel is different from a cassette, and you can avoid confusion by distinguishing them. Assuming that what you have really IS a freewheel, here's my advice...

A freewheel threads onto the rear wheel. As you ride it over the years, it gets tighter and tighter and tighter. Furthermore, if the freewheel is steel and the hub aluminum, they may seize together. This makes it VERY HARD to remove a freewheel. In order to do it successfully, you NEED a bench vise... there's no other way to do it reliably. Put the pronged/splined freewheel tool in the jaws of the bench vise, sticking straight up, and clamp it in place. Then put the wheel into the tool, laying flat. Now grip the tire on opposite sides, and rotate the whole wheel counterclockwise with all your might. After grunting and straining a few times, the freewheel should loosen and come off the hub.

Hope that helps! PS- You can spray a little liquid wrench or anti-seize stuff into the back side of the freewheel and let it soak in for an hour or two to make the job a bit easier.

+1
This is the best technique to use.

NB when attaching a steel freewheel to an Al hub beware of crossing threads. Reverse the freewheel (using the tool by hand) until the threads click into place then gently tighten. Pedalling action will tighten the freewheel firmly.

The vice is far and away the best way to go. However after my move to Ohio I haven't replaced my old vice yet, so let me tell you what I did for a particularly stubborn freewheel I encountered on my neighbors bike: secure the tool to the freewheel with the quick release, I put a long (26 inch) 1 inch automotive wrench on the tool (using the socket side) and put a length of pipe on the end of the wrench for an addition two feet of leverage, the hard part is finding solid pipe wide enough to fit over the 1 inch wrench head. With an assistant holding the wheel steady, slowly apply wheight onto the end of the pipe. I was worried about breaking a spoke doing this, but after inspecting the wheel didn't find any were busted.

The freewheel on my bike comes off about once a month so just the automotive wrench has been all I've ever needed for mine.