Droid Incredible - Gmail Push - Can I add my hosted Email address to Gmail on my Droid?
When you add your email account through the Gmail app (Menu &rarr; Accounts &rarr; Add account), you'll usually need to use your full email address for the "username", rather than just your internal username.

In addition, if your company uses single sign-on for Google Apps (that is, if the login page for your business Gmail is not on google.com and doesn't look like the Gmail login page), you may need to set up a separate password for accessing Gmail from a non-web interface.  For this you'll need to talk to your IT department.