I would use a RR501 Transceiver as they are better than the TM751 if you must use one.
I (and I'm sure plenty of others here, also) agree with Dan's recommendation.
However, since you PAID for a TM751, and X10 didn't send it, I think your first option should be to try and get X10 to send you the item you paid for.
However, if they don't seem to be responsive to that request (if you can't get through to them on the phone, others have reported fast responses when posting to their Facebook page), you should get your credit card company involved right away. If you paid for something they didn't send, and they don't seem to be willing to send it (or to refund your money for it), then contest the charge with your credit card company. They can pressure X10 to either correct the issue, or give the refund. In the case of a refund, the credit card companies will often grant you the refund, and THEN chase after the vendor to get their money back.