Uncheck Generate automatic URL alias problem When I add new content I uncheck generate automatic url, but below, the Url Alias text field where I want to add custom url is still disabled. I'm not able to write my custom url there.
Check if "Path" module is enabled.  If not, enable it.  I think it is enabled by default.

And go to admin/people/permissions and check "Administer URL aliases" under "Path" section.