This is dealt with extensively in manual and wiki.
As Reincaster explains, it's accomplished by putting FXL before the Cab block.
This means the Output 2 (jacks), not balanced) is activated. Because the signal flows to Out2 before the Cab, that signal has no Cab simulation. And can be used to feed the Mesa and cabinet. You can fine-tune the signal using Global EQ 2. The level is adjustable with Output 2 on the front panel.
After FXL the signal goes through the Cab block. This signal comes out of Output 1 (balanced XLRs, or unbalanced jacks), and has the Cab in it. You can fine-tune the signal using Global EQ 1. The level is adjustable with Output 1 on the front panel.
This means you can enable Cab simul. in Global. It will only work on the Output 1 signal.
Regarding Power Amp Simul.: you will want this set to On in Global. Otherwise the signal through Output 1 won't sound good.
But it also works on the signal coming out of Output 2. This is not a problem when using a neutral power amp. You may or may not like this with your Mesa, just try. If you don;t like it, you can use the method explained
here. Which involves using parallel paths and two Amp blocks, with one using power amp simulation, and the other not.
