I have NR3030 and also struggled with making CEC work. For my receiver, the 32910 control code did the trick (same as what shadow mentioned above). In NR3030 manual, they mention that you can also try 33101/33501/31612 codes - all to control a CEC-compatible BDP/DVD. Although, maybe your receiver is using different control codes.
I took a look at SR507 user guide (from their website, starting on page 84), and they mention some other ‘Remote Control Codes list’ for your receiver, maybe you need to locate that list / manual. If you don’t have that list, maybe check out NR3030 instructions, as they have quite a bit of codes listed there.
Although, saying that, I have just basic stuff working, and if I get stuck (not that often), I use Yatse on my phone.