If the light turns on/off via your Pronto and your IR Transceiver, the X-10 commands are getting through to your power line. The X-10 CCF file on this web site does have the dim/bright commands learned into the buttons labeled dim/bright.
I suspect that the type of X-10 receiver you have may be a non-dimming type. What is the model number on it?
The other possible problem is that the buttons in your CCF labeled dim and bright somehow lost the learned IR codes. Check in ProntoEdit to make sure they are still there. Also, when you press the button on your pronto you should see the IR Transceiver LED light.
Robert
RCI Automation
rciautomation@compuserve.com[Link: ourworld.compuserve.com]