If you are transceiving House Code A on both your CM15A and a TM751, then you are most likely getting powerline collisions, since both units will respond to your RF remotes and attempt to send commands through the powerline at the same time. I don't think your issue represents a software problem in AHP or the CM15A -- the CM15A should act as a transceiver for House Code A (if you configured it correctly) even if you don't add any modules to AHP.
First, do a full reset of the CM15A -- unplug it from the USB and power, remove the batteries, and let it sit for a few minutes. Now, plug it all back in and configure it to transceive House Code A: Start AHP. Under Tools > Hardware Configuration, select "Specific" under Transceived House Codes, then check the box for House Code "A". Click the Update Interface button.
Unplug your TM751. Now, with the CM15A as your only transceiver (and still connected to your computer), you can test your RF range by trying out your remote(s) from various locations in the house. Make sure your remote(s) have fresh batteries. If you don't see your lights turning on/off as expected, use the AHP Activity Monitor to see if the CM15A is receiving your RF commands. If the activity monitor shows RF commands but no powerline commands, then make sure you have the CM15A set to transceive House Code A. If it is properly configured, but you still don't see powerline ccommands, you may have a defective CM15A. If you see RF commands being received and powerline commands being sent, but your lights are still not working, then you may have a signal problem on the powerline or a defective CM15A. Try unplugging the CM15A and plugging the TM751 into the same outlet, and see if that works. If it does, then you may have a defective CM15A. If it doesn't, then you have signal problems on the powerline.
If you are not getting RF signals to the CM15A, you can use one of the many well-documented mods to increase the RF range on the CM15A. The simplest seems to be attaching a 9.5" wire (a cut-down coat hanger seems to be the implement of choice) directly to the existing antenna -- no cutting or soldering required. More extensive mods include removing the existing antenna and attaching a coax f-connector to the case, which allows the mounting of an external antenna. Search the forum (CM15A range) for additional details.