bench43,
You have some options.
1. Do the HTIB speakers use bare wire connectors at the speakers? If so, then you can connect those speaker wires into the back of the sub in the left/right speaker-level jacks. That will send a signal to the sub and let you know if it is working at all. You'll either want to bypass the subs crossover (with the switch) or set it as high as it will go (the similar sub manual shows a dial going to 120 Hz).
2. If your HTIB has any sort of RCA output jacks (e.g. for a VCR), you could connect that RCA jack to one of the line-level RCA jacks on the back of the sub. That will also send a signal to the sub.
3. You can use a minijack-to-RCA splitter cable (can get at Radio Shack for cheap) and plug anything with a headphone jack into the line-level inputs on the sub - iPod, HTIB, whatever. You might need to turn up the volume on the component with the headphone jack, but that will also send a signal to the sub.
4. If you have components that have RCA output jacks, like a DVD player, VCR, or tape deck, you could connect those RCA outputs to the line-level RCA inputs on the sub.
Whatever you do, I highly suggest that you start with the volume level of the sub set to the minimum and work your way up.
Adam