When you plug the SanDisk into the computer, go to "My Computer" and you should see the SanDisk there along with the drives. It'll say how much room is left on it, if it is a 16gb flash, it may say something like 15gb free. The software to encrypt is probably on the 1gb that is used already assuming you haven't formatted it (in which case you will have deleted the security software)
It should automatically load the software unless you have autoplay disabled. Or if your security is disabling and auto execute files from the SanDisk.
Remove it from the computer, go to the control panel and there should be an option labeled "AutoPlay". Click on that and enable or make sure it is able to use autoplay. Then plug the flash drive back into the computer.
When it comes up to show files, etc. see if there is an option to run and .exe file. If there is then that is your software. If not, then you may have to download from the Sandisk website - it is a file called Cruzer Lock.