Minted requires Shell Escape to be enabled. This is a potential security hole, so it is not enabled by default in Texpad. To enable Shell escape open Preferences, switch to the Typesetting pane and check the -shell-escape box.

By default Texpad invokes LaTeX in such a way that all intermediate files are kept in the .texpadtmp directory (a hidden directory at the same path as the root file). Unfortunately it appears that this is confusing the minted package because the intermediate files are not where it expects them - we do not know of any way to invoke Minted in such a way that think there is any way to use minted with Texpad's hidden directory, however you can turn off .texpadtmp.

Opening preferences, choosing the typesetting pane, and unchecking "Hide Intermediate Files" disables intermediate files and allows Minted to operate normally.