See the FSL project page. To use FSL, you’ll use the module tool.

You can see what versions are available by using:

[me@login01 ~]$ module avail fsl

------------------------------ /opt/modulefiles -------------------------------
   fsl/6.0.7.8

Use "module spider" to find all possible modules and extensions.
Use "module keyword key1 key2 ..." to search for all possible modules matching
any of the "keys".

To load a specific version, you would use:

[me@login01 ~]$ module load fsl/6.0.7.8

while the "fsl" wildcard will load the default version, fsl-6.0.7.8 in this case.

You should now be able to run FSL commands:

[me@login01 ~]$ flirt -version

FLIRT version 6.0