To allow sign-in access at select sign-in stations, select the opportunity on the "Opportunities" tab and click "Edit." After that screen displays, click the "Sign-In" tab. At the sign-in tab, you can set-up the sign-in station(s) and customize a special message for your volunteers. When you are done, click "Finish."
Then, place volunteers for that opportunity. Now, sign-in is ready to go!