I am having this issue too. For some reason after a few days, Jive seems to always go into my "inactive" excel add-ins. It is an annoying process, but the way I correct it is by clicking File--> Options. I then select "Add-ins" from the list. At the bottom of that page you'll see a drop down list. You can select "COM add-ins" and click "GO". A pop-up box will appear and, if Jive is installed, you should be able to select the Jive for Office plugin and then click "ok". Then you will see your Jive Ribbin at the top.
Like I said, this is an annoying solution, but it is what I have to do right now. Hopefully this gets fixed in one of the next few releases.