I don’t know that there is an option to create a watch expression directly from Xcode 4’s code window. If there’s something in the code window you’ll want to add an expression watch for, I think you’ll have to copy that code to the clipboard.
Then while debugging, make sure the debug area is being shown. It is the bottom center view in Xcode. You can show this area by clicking the appropriate view icon from the debug view.
Then in the lower left of the debug area, you have your watch list. You can add an expression to this list by right-clicking and selecting Add Expression:
Right-click to add an expression http://blog.roughfalls.com/non-wp/img/xcode4_addexpression.png
After you select this option, a dialog appears. You can paste the expression that you copied to the clipboard into this dialog, and click “Add.” Watch expressions are denoted in the list with a small “E” icon to their left.