Go to your Game Mail, click on the header of the message you want to respond to, scroll down until you can see that message, check the "Reply" box, press Submit, type in your message at the top, and then press send.

It might sound obvious if you've already figured it out, but for my first couple of days it was beyond me to actually scroll down. xD

If it has sent, the message will appear in your Sent Mail Box.

If you still can't send anything, I'd recommend posting something in the Bugs section of the forum so that it can be addressed and fixed.