You can find the latest US Payroll updates under the "Related Resources" section of KB 45424: Sage 300 ERP Payroll Updates Downloads.

Please look for the tag (2014 Q4 SR1) to differentiate the new January 2015 updates from the previous December 2014 updates. If you are logged into the Knowledgebase, you can also access the January 2015 updates by visiting the individual KBs.  

KB 56590: Download Sage 300 ERP 2014 US Payroll Update 7.1F (2014 Q4 SR1)
KB 56593: Download Sage 300 ERP 2012 US Payroll Update 7.0P (2014 Q4 SR1)
KB 56594: Download Sage 300 ERP Accpac 6.0 US Payroll Update 6.0U PU1 (2014 Q4 SR1)