Hi,
Goto SPRO -> Materials Management -> Valuation and Account Assignment -> Account Determination -> Account Determination Without Wizard -> Define Valuation Classes
or execute tcode OMSK.
Also Go through the link:
http://help.sap.com/saphelp_46c/helpdata/en/1e/1a39516e36d1118b3f0060b03ca329/content.htm
Regards
Shishir