iMessage for iPhone and Mac

Hi everyone,

I've been having problems with my iMessage on both my Macbook Pro and my iPhone 4S. I'm logged into both accounts, but my mobile number is not showing/cannot select.

Does anyone have any suggestions on how to fix this? How do I make sure that my cell phone number is on my Apple ID?

I don't know if it makes a difference, but my phone is American, but I am currently in Belgium (so my phone is in airplane mode). I haven't had a problem until now, even after having been in Tanzania and several European countries.