The enlarger appears to be an Omega B-8. (it also might be an autofocus B-7–the picture isn't that clear). I bought one [B-8] about seven years ago and think it's a very good enlarger; probably the best medium format enlarger Omega ever made. It's the fourth enlarger I've owned (over 50+ years) and I'll never need a better one.
To answer your questions:
1. the 50 mm lens is for 35mm negatives, the 75mm for 6 X 6 negatives, from 120 film. The enlarger will work with up to 6 X 9 negatives, but that size needs a 90–105mm lens.
2. the 50mm lens goes with the flat lens board; 1he 75 with the deep lens cone. Both should have round lens discs attached, to which the lenses are attached.
3. When you say "2 different sets of condenser glass, one set is bigger then the other" I hope you mean four glass condensers. The two large ones are the main condensers which fit inside the aluminum tube just above the negative stage (where the negative carriers go). They go flat side out; there should be an aluminum spacer between them, which MUST be used.
The two smaller condensers are supplementary, used alone; the thinner one for the 75mm lens, the thicker for the 50, none at all for a 90–105mm lens used for 6X9. They go (flat side up) just under the lamp housing (which comes off if you loosen the set screws). They should have one flat edge, which makes them easier to remove. My 50mm supplementary condenser is an after-market one. not made by Omega, without that flat side and requires a small suction cup to take it out.
The thick condenser is really needed for the 50mm lens. The thin one may not be essential for the 75 (and certainly isn't for an 80mm lens–I used an 80 for years without one, before getting the VERY hard to find thin condenser.
You can find more information about the enlarger here, on the KHB Photografix site (click on "Discontinued enlargers" and then on "Omega B8":
http://www.khbphotografix.com/omega/index.html
And on Harry Taylor's website:
http://www.classic-enlargers.com/omega_b22_enlarger.htm
Harry also has a "Help Forum" which is very useful:
http://www.classic-enlargers.com/cgi-bin/yabb/YaBB.pl?board=general
Unfortunately, he is quite elderly and seems to have stopped responding to questions several months ago, but the archives are very valuable for anyone interested in Omega enlargers.