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If you are trying to edit the Main Menu block from the Default Template then you can not do it. This is sort of a safety feature for the XOOPS core etc (as far as I can tell). But here is how you can do it:
You have to Clone the Default Template first giving it a new name of your choice.
Then you have to set the one you just cloned with the new name as your Default Template in the Administration "General Preferences".
You should see a drop down for setting the the template. It should have Default and also the name of the New Template you just cloned and named.
Choose the new one and then scroll down to the bottom of the page and click on Go.
Now go back to your Administration, Sytem, Templates and click on the "list" for the new Template set you made. You should be able to edit the Main Menu block now.
Hope this helps.
Quest