The cross gcc tools was expected to be installed in /opt/nios2 dir. But you may install in any other dirs. Then setup the PATH for the cross gcc, you can add a line at the end of file ~/.bash_profile (or ~/.profile on Debian/Ubuntu) ( the file is hidden, you have to use "ls -a" to find it . For "gedit" use open Location, and enter the file name)
After you change the .bash_profile or .profile, you need to logout and login again. So that it will be in the PATH when you login.
Check with, echo $PATH
It should display like this, with nios2 path at the end, /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/opt/nios2/bin
To verify the cross gcc, try out, nios2-linux-uclibc-gcc -v
It should display the info,
Reading specs from /opt/nios2/lib/gcc/nios2-linux-uclibc/3.4.6/specs