I was able to fix this problem by including my @gmail.com address to my checklist of ways I can be reached (for what it's worth, this is my apple ID address/username). Activation code went through, and then I unchecked the @gmail.com addresses, because I only want to receive texts/iMessages to my phone number.
Additional notes:
The problem happened on my Late 2009 iMac + iPhone6. After that was activated, I was able to hook up my Early 2013 Macbook Pro (Retina) without checking the @gmail address. However, it was still checked on my phone.
I'd recommend turning iMessage on/off on both devices, including more than one address/means to receive a message, and making sure you're on the same wifi network. If your home router broadcasts on 2 different channels (5ghz/2.4ghz), make sure you're on the same one.
Edit: http://www.macrumors.com/2014/10/21/test-message-forwarding-no-code/