I just got the MoP Beta a day ago and with much joy started to download it. However I was running into a few problems getting it to download. So In case you are having problems, here’s a few things you might want to try:
1) Make sure you are running the beta launcher as an administrator and the agent.exe program as well. If you can’t find it bring up your task manager, right click on the process to bring up properties, set it to run as administrator and then terminate the process and relaunch the beta downloader.
2) If that fails, it might be due to a corrupted download. So delete the c:\programdata\battlenet folder and the setup files and launcher will be re-downloaded the next time you start the beta download.
3) Another strange error occurs when your agent.exe is tricked by old unused internet connections. Open your networking properties in your control panel and check to see that there aren’t any up-used dial-up connections or vLan connections listed.
4) If all else fails try downloading the setup file again in case it was corrupted.
5) Blizzard also advised keeping your graphics drivers up to date as the set up will complain about it.
6) Lastly, make sure you don’t have a firewall program blocking the downloader.
Hopefully one of the tips helped if you are having problems getting the beta up and running. There are also lots of useful posts on the official techinical support forums so don’t give up.