Ruckus won't let you buy a URL Filtering license for your Unleashed network unless you are the original purchaser of the APs.
If you have a used Unleashed AP and would like to enable URL Filtering, you may use the procedure here to apply a license.
This patch should be uploaded as a Preload Image (Admin & Services > Administration > Upgrade > Local Upgrade > Preload Image).
The upload process completes the patching; no upgrade will be offered. Simply wait a few seconds after the upload, and the license should be enabled.
Although the patch can be directly downloaded and used, I recommend either creating the patch yourself or decrypting the patch to verify it does only what it should.