Adding a sidebar within vBulletin 4.2.x requires an additional add-on that you can download from the vBulletin.org downloads. You will need to be a verified licensed user before downloading.
Once this add-on is installed, you will want to go to the options page under Settings > Options > Sidebar Anywhere by BOP5.
Once that page has loaded, scroll down to the Custom Scripts & Templates section.
You can add an additional html block by going to Forums & Moderators > Forum Blocks Manager.