Welcome to the forum, Whitney!
The way to hook that sub up to that receiver is to, instead of connecting the receiver directly to your T15 speakers, connect the sub in between. That's because your subwoofer has speaker-level inputs and outputs, which is a great thing for you because (as you said) your receiver doesn't have a line-level (RCA jack) output for a sub. This is covered under the "Option #2" connection diagram in your subwoofer's owners manual (link to PDF
here).
So, you want to:
1. Connect the left/right speaker outputs on the receiver to the left/right speaker inputs on the sub using speaker wire
2. Connect the left/right speaker outputs on the sub to your left and right T15 speakers
3. Adjust the "Low Pass" dial on the sub to get a good blending between the sub and the speakers. Based on the specs of the speakers, I'd start off with 80Hz and go from there.
I hope that helps, but please just let us know if that isn't clear.