How do I open Outlook .msg files on my iPad

You don't need an additional app. You can either sync your email messages to your iPad from your computer - instructions are in the user's guide as well as here:

http://support.apple.com/kb/HT1386

or you can set up your iPad to connect directly to your email provider - instructions also in the user's guide and here:

http://www.apple.com/support/ipad/mail/

If your email service is an Exchange server, I'd recommending setting up your iPad to talk directly to the server. Then your mail, calendar and contacts, if you wish it so, will synchronize automatically without you having to sync with your computer.