are you trying to connect your iphone to your mbp to access the web?
if so, what i do with my imac and my iphone follows:
1 with airport on, open sharing under internet & network preferences
2 highlight internet sharing (don't check the box yet)
3 choose an option for 'share your connection from' - my imac is wired to my modem, so I opt for ethernet. if your mbp connects to a wireless router, i would choose airport.
4 check a box for 'to computers using:' here the choice is airport
5 click the 'airport options' button > here, whether or not you've created the adhoc network through any other means, it wouldn't hurt to fill the data fields again. on at least one occasion, the info here (password) was different than the password i created for the same ad hoc network i created through airport's taskbar menu > click ok
6 check the box for internet sharing
7 confirm that you want to start sharing
8 the icon on the taskbar should fill in with an upward pointing arrow
9 on the iphone, open wifi settings and choose the ssid name for the network and input the password (if you opted so for your network)
that should grant you access on the iphone to use safari or otherwise check email/im/text/map something at wifi speeds
hope that helps
good luck
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