If it's anything like the Podxt, which I'm pretty sure it is, and speaking from my experience with the Podxt pro, just tweak tweak tweak tweak tweak, test in a mix, tweak tweak tweak, remember its not a real amp, so don't treat the controls as such, forget what the controls are supposed to do on a real amp, tweak tweak, test in a mix, tweak tweak, try a tubescreamer, tweak, get frustrated at the fizz, tweak tweak, roll off some highs with the eq to get rid of some the fizz, tweak, realize it's not crisp anymore, and sounds muffled, bring some highs back in, tweak, pull some of your hair out, tweak, save the tone, take a break and do something totally unrelated to music, come back the next day and start over after realizing the tone you had is nowhere near what you wanted, tweak tweak, switch amp and cab models around, try to avoid diming any of the amp controls, test in a mix, tweak, and hopefully by this time you have something usable.
Or you could try searching for and downloading some of the user created tones posted in these forums and use them as a starting point. Most metal Podxt users seem to prefer the big bottom amp model with a variety of the 4X12 cab models, with either one of the sm57 mic models or the 421 model. Using the tubescreamer stompbox model is highly suggested, unless you have a real tubescreamer to run out in front, then you can use the EQ+boost stombox for further tone shaping. Not necessary, but helps.
The best tone I've yet achieved with the Pod, in my opinion, was by using the EQ+boost with the Engl Powerball amp model, with the Marshall 4x12 25 (I think its called) cab model, with the sm57 off axis. Tweak tweak.