How can I generate UML diagrams from Xcode4
Solution 1:
Try Omnigraffle This has templates for UML diagrams.
If you drop an Xcode4 project file onto its dock icon, it will generate a UML diagram from that.
What it doesn't do is generate code from diagrams.
Solution 2:
Search for "Xcode Design Tools" in Help. That will show you a tutorial that will walk you through creating a diagram.
EDIT >>>
It looks like the instructions are actually for XCode 3 and that UML Modeling was removed from XCode 4. The only modeling I could find in XCode 4 is Data Modeling: File>New>New File... and then look under 'Core Data'.
Class Modeling alternatives for Objective C
You might also want to check out Doxygen.
Solution 3:
You can use yEd ... not the best solution and does not generate graphs as OmniGraffle can do but at least it can export in SVG and use GraphML files to store the data. (and it is fee)
http://www.yworks.com/en/products_yed_helpresources.html