The BO XI 3.1 software is Complete itself. 
Complete with some bugs 
…so you need to install Fixpacks/Service Packs in order to address the issues/bugs which are identified after the release of major product.
I hope this link will help you in finding the download area. There would be different areas for downloading the major products and SP/FP’s.
Currently, SP3 is the latest Service Pack released till date. And FP2.8 is the latestet Fixpack.
So I would suggest installing SP3 on top of XI 3.1 to be on latest (Complete) patch level. 
nicholas (BOB member since 2008-07-31)