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You need to do two things:
1. Go to Administration Menu and select "Permissions" from the Forums popup Menu. From the 2nd of the two drop-down boxes underneath "Permission management tools" select "Can Edit" and set permissions by user-group.
2. In the module's Preferences, scroll down to the entry marked "Time limit for edit a post". Set this to what you want (in seconds). Enter "0" if you want no time limit.
Does this help?