On a new laptop (Yosemite) I had trouble. It saw the Airport, rejected the password and gave me an exclamation mark. I can tell you I made a few exclamation marks myself!!
My old laptop was still connecting. New one didn't. I tried a lot of things. What worked in the end was deleting old 'locations' for wi-fi and making a new one.
- Open System Preferences. (Under Apple Logo)
- Open 'Network' logo
- Select Wi-Fi
- Select 'Location' tab
- Select 'Edit Locations'
- Make new Location with new name
- Delete all other locations
- Re-start your computer and Airport Express (but you may not need to. The connection should be instant)
This worked for me, and I just remembered it has worked for me in the past. I think a new Wi-Fi location gives you a fresh set of preferences or something so you don't have to get inside your Library and delete files.