I don't know if this is your problem, but sometimes lowers, particularly cheaper ones, are out of spec. It sounds as if it simply is out of spec.
I will note that some polymer lowers that I have experience with like the cav arms lowers tend to be a little tight in places initially and wear in a bit with use. that is different than simply not fitting at all though.
As to ideas, I'd say there are a couple of possibilities.
1) try a different catch, although it sounds like this wont make any difference because it sounds like it is not a matter of the catch being out of spec. (you might also check your mag catch in another lower).
2) Contact the seller and/or manufacture.
3) Carefully remove material (in theory from the mag catch or the lower) until it can fit. I agree with the above advisos about 1. not using a dremel and 2. researching it more before you do something you cannot undo.
4) Leave it un-built, keep it or sell it and learn a lesson about buying cheap plastic AR lowers. I have learned my own lesson about cheap lowers, having previously gotten two forged lowers that were out of spec, one very seriously so, I've learned a bit of that myself.
A quick google search turns up other instances of parts not fitting in real well. See e.g.
http://www.ar15.com/forums/t_3_4/601444_NFA_lower__mag_release_button_will_not_go_in_.html&page=1