it's been a couple months since i've used one (and quite a few more months since i posted here last!)
the "light leak" photo, i believe, is where the negative carrier goes. it seals the gap up pretty good. and, IIRC, enlargers don't shoot all of the light through the negative. some leaks out here and there...
the "what's this" photo is the holder where you place the color filters. the ones i used were varying shades of red. the darker the color, the greater the contrast achieved on the print.
the lever that lifts the light box from the bellows allows you to slip in the negative carrier.
i have no idea where the lens goes. i've never disassembled/assembled one. my suggestion is, if you're like me and enjoy taking things apart, take it apart and look for a place that looks like it could use a lens.
the very last lens, at the bottom of the bellows, should be able to turn. that controls the aperture, and thus the amount of light coming through and hitting the photo paper.
oh, and you'll need a timer to hook up to the enlarger so you know your exposure time. or you could manually switch it on and off, i suppose, counting seconds in your head. but that's a bit more imprecise...
hope that helped a bit
tmm