Guitar --> HD500 --> 2:90 --> Cab.
I think he is reference slightly (as so the 2:90 manual says as well) that if you use a mono setup, you will be using one channel of the 2:90. The best way to treat the other channel is to not have anything hooked to it, turn the level all the way down, and turn the presence control all the way up. This is from page 5 of the manual. If you run stereo, then you just hook both sides up.
JMMP walked you through a stereo setup, and since the HD500 is stereo why not take advantage of it. If you run stereo you will need two speakers. One speaker for the left side and one speaker for the right side. This could be one cab with two speakers and each speaker has its own jack (stereo), or a 4x12 cab with one side (2x12) being one the left side and the other 2x12 speakers being the right side.
Just match up the ohms. There is some play in Ohms. On the back there are 8 ohm and 4 ohm jacks. Just find what the ohms are for your cabs and put them in the right spot.
My setup, I have a 2x12 cabinet, the Mesa recto cab. I bought a add-on that let me switch the 2x12 cabinet to a stereo setup instead of both speakers being wired together. Then I one stereo. In this particular cab they speakers are 16 ohm's per speaker. It is ok to run a 16 ohm speaker in a 8 ohm jack on the 2:90, but it is not ok to run a 4 or 2 ohm speaker in the 8 ohm jack.
If you need help with the speaker setup after you get some speakers, just post and we will help with the ohm numbers.