In Xcode 4
- Product menu > Manage Schemes
- Select the scheme thats having debugging problems (if only one choose that)
- Click Edit button at bottom
- Edit Scheme dialog appears
- in left panel click on Run APPNAME.app
- on Right hand panel make sure youre on INFO tab
- look for drop down DEBUGGER:
- someone had set this to None
- set to LLDB if this is your preferred debugger
- can also change BUILD CONFIGURATION drop down to Debug
- but I have other targets set to AdHoc which debug fine once Debugger is set