The iOS app has no convenient way to get from a main site to its Meta. You have to go to the full site list and search for the specific Meta you want.
Could a link be provided somewhere? My first thought would be for a tap or tap-and-hold on the site name at the top of the Questions tab to pop up a "menu" with a link to the Meta (see the Taskpaper iOS app for this behavior). The Meta should likewise provide a link back to Main.