Refer to page 18 of the Garritan's manual to get the correct installation instructions. Here is a link to the manual: http://www.garritan.com/wp-content/uploads/2013/01/PO4_Manual_March5.pdf
https://makemusic.zendesk.com/hc/en-us/sections/203979527-Setup-Installation
If you are running the 64bit version of Studio One, you will need to install the 64bit version of Garritan or it will not show up. Please try the above suggestions to hopefully resolve the issue that you are having. If the problem persist, you would need to contact Garritan for more assistance.