Here is one possibility that I ran into when I had the same issue:
I had started the BES install using the besadmin user. During the install, a reboot is required and then the install continues. After the reboot, I logged in with another admin account out of habit and continued with the install. I then had the same problem as the original poster and could not continue the install.
Finally, I rebooted and then I relealized my error and logged in with besadmin user. This time when the setup was continue, I was prompted for my excahnge credentials/ mapi settings and all worked well.
Hopes this 'doh' moment helps someone else.