Install
# chmod 777 /opt
# tar xvf gcc-3.4.6-2f.tgz -C /opt
# cd /opt/gcc-3.4.6-2f/bin

Set env
there are three ways to set: 
1. # export PATH=$PATH:/opt/gcc-3.4.6-2f/bin

2. # echo export PATH=$PATH:/opt/gcc-3.4.6-2f/bin >> /etc/environment 
   # source /etc/environment

3. # echo export PATH=$PATH:/opt/gcc-3.4.6-2f/bin >> ~/.bashrc 
   # source ~/.bashrc 

Check
#  mipsel-linux-gcc -v
when you see some words like below that's install successful.

Reading specs from /home/cpu/gcc-3.4.6-2f/bin/../lib/gcc/mipsel-linux/3.4.6/specs
Configured with: ../gcc-3.4.6/configure --prefix=/opt/gcc-3.4.6-2f/ --target=mipsel-linux --host=i686-linux --enable-threads=posix --enable-shared --disable-checking --enable-languages=c,c++,f77 -v
Thread model: posix
gcc version 3.4.6

Example
# mipsel-linux-gcc example.c -o example
# file example
example: ELF 32-bit LSB executable, MIPS, MIPS-II version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.4.0, not stripped