If you wish to test the StoreKit API or Apple Push Notification services, you will require an entitlement. The entitlements are defined in a plist format that is documented by Apple, and MonoDevelop has a file template for adding a new entitlements file to your project.
However, the current version of MonoDevelop only supports automatic processing of the entitlements for distribution builds. For development builds you must follow some manual steps.
First you must generate an xcent file, which is a proprocessed form of the entitlements plist.
Use the following command to generate the xcent file from the Entitlements.plist file:
/Developer/MonoTouch/usr/bin/mtouchpack \ --genxcent=yourappname.xcent --entitlements=Entitlements.plist \ --appid=HGWHD.foo.bar.baz
Where the appid is the full ID including the prefix defined in the provisioning profile.
In the project options dialog box, in the "iphone bundle signing panel" add the following command line in the "additional arguments" entry:
--entitlements=/path/to/the/processed.xcent
This dialog is shown belog.

For the distribution builds you can add the entitlements plist directly to the "custom entitlements" field in this dialog, and the xcent will be processed and included automatically. In future this will work for development builds too.