It's not really clear how much you know so I'm going to try a very basic approach, please don't be offended.
I'm assuming the NSTextView is in a external nib/xib and is not created programatically.
If so open up your nib/xib in Interface builder and click on the offending NSTextView element.
Then hit Cmd-5 or Tools->Connections Inspector
From here you can see which Outlets/Actions to look for in the actual source code.
Most likely you're problem is manipulation of a "Reference Outlet" object, or in a "Sent Action" function.
Once you have an idea of which method(s)/variable(s) you need to watch you can open up the source code for those parts in Xcode. You can then set a break point at any line by click on that line number on the left hand side of the editor window.
Click on Run->Debugger. A new window should popup, click "Run and Go" in the toolbar of that window. Your app should run as normal until it hit's the break point you set in your source code. when the break point is hit you can mouse over variables in your code in the bottom half of the debugger window. The current values of the variables should popup allowing for easy inspection.
Usually you can gain the same information by using the Top Right panel of the Debugger to inspect the variable table.
When your done with that break point click "Continue" at the top of the debugger window and your program will execute till completion or the next break point.
This should get you started with debugging under Xcode.
Their are different kinds/uses of break points and all kinds of additional fancy stuff you can do in Xcode.
For more information check out this awesome CocoaHeads video.
Good Luck,
Brian G.