First you need a sequencer which is able to use the new Rewire 2 protocol. I use Nuendo 1.5 by the way.
When you enable Rewire on the sequencer you should see several Rewire audio channels. Those channels will send Reason's outputs to your sequencer's inputs. So you can EQ them or add some effects. You can also send every Reason devise to a seperate output if you want to get total control over it's sound.
No try to start your sequencer and load some loop into the REX-player and press it's 'play' button. You'll hear the loop in perfect sync with your sequencer. Try lowering the tempo and you'll hear the loop go down in tempo as well. Controlling the tempo from Reason's own sequencer or Nuendo for example doens't matter, both sequencers are in sample accurate sync.
But if you want to let your sequencer trigger that loop just insert one D0-note (that's a D note in the 0, zero octave) on the first beat of the bar. When starting playback you'll hear that the REX-player is being triggered by your sequencer now. So you'll won't have to press 'play' on the REX-player anymore.
If your REX-loop is just one bar long, you have to insert a D-0 note on every bar in your song.
You can also trigger the REX-player by using higher notes (just find them using a MIDI-keyboard). So now you can also get control over the timing of every slice.
Using a combination of just triggering the whole loop and trickering the slices one by one, makes your music very flexible just because you're using sliced up REX-files.
I can't wait to get my hands on ReCycle 2.0 which can slice stereo-files.