Make sure your VM is on a bridged connection to your Mac (not natted) and follow these instructions to put MAMP into online mode:
"To allow external connections to your server you have to restart WAMP in online mode.
Left-click the WAMP icon and select Put Online.
Wait for the icon status to change to white again.
Enter your IP into your web browser bar and hit enter.
If you get the same forbidden page as before hit F5 to let the browser load the newest version.
If it's still not working you probably have to enable port 80 forwarding in your router configuration."
found here: http://www.tagbytag.org/tutorials/getting-started/web-design-software/wamp-guide
You can skip the last port forwarding step since your VM will be accessing from inside the network.
edit Oh, sounds like you got it. This might be helpful to someone else so I'll leave it.