"Auch diese beiden kommen ab und zu mal vorbei."
I have hard time understanding (and there for learning) why does "ab und zu mal" translates to "now and then"
"Ab und zu" is a fixed expression. You can't translate it literally. It means "now and then", "off and on", "from time to time". For a Spanish translation, see: http://goo.gl/tjJRv
For a fun way to remember the expression "ab und zu", I recommend a song from the German version of Sesame Street :) : "Kekse isst man nur ab und zu" (You eat cookies only now and then). http://www.youtube.com/watch?v=hPwbfolmNMM
"Mal" literally means "once". However, in this sentence it is used as a so-called modal particle. It doesn't have a "real" meaning here, it's just used to make the sentence sound more casual. You could also omit it without changing the basic meaning of the sentence. Modal particles are complicated phenomena and it's not always possible or even necessary to translate them. Duolingo's English translation just omitted the word "mal". For more information on modal particles, see: http://coerll.utexas.edu/gg/gr/mis_04.html
Edit: Too late :)