Hi,
You can find description of field mapping here: http://rc.crm.dynamics.com/rc/2011/en-us/on-prem/5.0/help/ui_cust_mapping.htm
Keep in mind that the option sets have to be equal - in best case you use a global option set for both entities!
Best regards,
Andreas
Andreas Buchinger
Microsoft Dynamics Certified Technology Specialist
MCPD: SharePoint Developer 2010