1. ASSERT: make sure that you have only 2 spare application slot (given your quota is 3: have 1 applications) and none of the used apps is a jenkins type app
2. ASSERT: make sure you have a "Dynamic Web Project" in your eclipse workspace
3. EXEC: lauch the OpenShift Application Wizard:embed the jenkins client to it and create the jenkins app for it.
4. EXEC: on the last wizard page: uncheck "Create new", choose your "Dynamic Web Project" and hit "Finish"
5. ASSERT: now you have no spare application slot, quota is full (given qutoa is 3 you now have: 1 pre-existing app, 1 new app, 1 jenkins app)
6. EXEC: log into your jenkins instance and see if your app is building

Result:
The jenkins job is waiting for a slave to get available

The reason is that jenkins needs an additional builder application. You already reached your 3 apps quota and jenkins therefore cannot build your app.