The Install button in Disc Manager is not active if all of the content is already installed.
Revisions are managed by the Yellow update disc. After updating your computers database the update will ask for needed DVDs. If you skip inserting the DVDs when requested and then go to Disc Manager the Install button would be active (because you do not have all the content installed) But if you have all content installed then the Install button is inactive.
Note: many times you will receive a DVD and NOT be asked to insert it. This is because the changes on the DVD were not for manuals in your subscription. You need the new DVD in case you ever wanted to re-install but there is no new or revised data for you to add with this revision.