How to change back to the console when stucked by the desktop environment

[Yubin Ruan]

Hi,
I just tried FreeBSD-10.3 stable and get stuck when trying to use the 
Mate desktop environment. I use Slim as the login manager. But when I 
try to login, with correct username and passwd, Slim tell me that
     "Failed to execute login command"
(Any idea why would that happen?)
So I try to switch back to the console use "Alt-F1...Alt-F8", which is 
suggested by the doc. But that doesn't work. Why ?

How can I switch back to the console to tune the system?

Regards,
Ruan