open up a command prompt and type

It should spit back something like

/bin/su

If it doesn't, type
or
If it isn't there, try the search fucntion. Click on the Kmenu and go up to find files and search for su. If it's there, you will need to make a change to your environment. If it isn't, you will have to login as root and install it from some medium, or online. But first see if it is actually there or not.