What can be done to use iMessage when it is stuck on "Waiting for Activation"
A few of the things that are under your control are as follows:


Undo the iMessage settings, power off the device and try again in 5 minutes.
Check that iCloud isn't having system-wide problems
Same as step one, but wait 20 to 200 minutes. (sometimes the problem is in the network or the activation servers or the SMS network to tie your phone to the Apple system with the one time activation message and waiting is easier than working with all of the above parties)
Rather than wait - work through the steps to Troubleshoot iMessage and FaceTime Activation
Contact Apple for support


It's hard to guess what exactly is wrong, but you also can edit your question to list what steps you have done to troubleshoot this and perhaps someone can guess or point out a missed step.