If there is a way to get them to work, besides getting a receiver with pre-outs, which is a waste of money, not sure.Beside your trying to get some nice studio monitors to work with a really low cost sub-woofer, part of me thinks the Polk Audio will hurt (interfere with) the JBL's sound quality.
Return the Polk audio and do not use that external 5.1 sound card.
Plug the E10K into the MacBook Pro's USB port.
Use a cable like this to connect the E10K to the JBL LSR305.
(3.5mm stereo plug to dual 1/4" mono plugs)
Monoprice 5" studio monitors, $136, normally $171
Creative Labs Sound Blaster Z sound card, $100
So total could be around $250, well under the $300 budget you had for audio.