Problem... wireless card / USB dongle drivers and firmware appear to be correctly installed but the wireless password is not accepted.
My setup: Dell Inspiron 5100 + TL-WN721N wireless USB adapter + Maya XFCE
Here's at least a possible partial work around solution that established a wireless connection for me...
Go into your wireless network connections, edit your wireless network, deselect 'connect automatically', and select 'Available to all users'. Save and close. Then left click on the network icon (bottom right) and select your wireless network. You should get the popup window requesting your network password. Enter your password and hopefully you'll connect.
The above steps established a connection for my setup. I then did some additional testing to discover that the problem is not really solved. I rebooted and the USB dongle was not automatically recognized, so again no wireless connection. If I remove and re-insert the USB dongle the dongle is recognized and I can re-establish the connection. I also deleted my wireless network listing from Network Manager and rebooted with the same results. Except this time when I re-inserted the dongle I was prompted for a password and my wireless network entry in Network Manager was automatically created including 'connect automatically'. When I now reboot my laptop with the USB dongle inserted, still do not get a wireless connection. When I remove and re-insert the dongle the wireless connection is made automatically including my password login. So something within my setup is still not correct.
Before I discovered the above partial solution, I spent several hours trying to get Maya 13 XFCE running on my older Dell Inspiron 5100 using the internal Broadcom BCM4306 (legacy) mini-pci wireless card. Once I got this same point where the password wasn't accepted, I opted to try my TP-Link TL-WN721N USB dongle (listed as Linux compatible). I re-installed Linux Mint with the USB dongle installed, but the USB dongle with the fresh install had the same problem, ie, the wireless dongle was being identified (seems the firmware and drivers were loaded), but my wireless network in the Network Manager panel was not listed and would not connect. After manually entering my wireless info and password, the connection could not be made and the request for password kept re-appearing.
Finally today I read a post that suggested the above solution which partially worked for me. Suspect this would also have worked with my Broadcom card after I had discovered all of the hardware tidbits... finding and loading the BCM legacy drivers required for the BCM4306, commenting out BCM 'blacklist' in the config file, etc.
Given several of the posts I read today suggested that an older version of linux mint (2.x kernel) might not have this problem and/or a Linux Mint version other than the XFCE desktop may work. So I may try that next. In the mean time I can get connected wirelessly and my wired connection has worked flawlessly.
Thanks again to the Linux Mint team.
Hope the above helps anyone else having this problem.