Newbie question - mixing stereo tracks + 4 channel from Zoom

I’m new to Audacity and need some help. I’ve spent a lot of time looking at the manual, but I just can’t figure this problem out. I recorded a rehearsal of my band with my new Zoom H4N. I took a line out from my mixer and used that for my vocal. It went in on the 4 channel mixer setting on the Zoom. And I used the internal mics to record the ambient sounds of the instruments. I got two files off the reader card - one has the vocals, one has the instruments. So far so good. I can’t for the life of me figure out how to mix it down in Audacity. As per the manual, I opened the two tracks simultaneously, and I think that mixed the two files together, but I can’t seem to get to the separate recordings and adjust the eq separately. Please help? I’m using Audacity 2.2.2 and Windows Vista.

I don’t have a Zoom H4N but I guess that when you import, you see two stereo tracks. Is that correct?
See here for how to split a stereo track into two mono tracks:

That looks pretty helpful, thanks!