You have to purl into the back of it!
Just as if you were going to
knit into the back loop if doing it flat, you have to do your purl
through the back loop. I find it a bit annoying so I always just
switch back to "regular" knitting when I am ribbing in the round. I
have read however that some people go ahead and just make the
stitches twisted when doing ribbing with combination knitting because
it can make a tighter rib. I am planning on trying this on my next
project with ribbing!