Actually am3 processors can work on am2/am2+ motherboards, but can be dependant on bios update, being that board is cheap biostar it may not have appropriate bios update.
Article on socket am2 http://en.wikipedia.org/wiki/Socket_Am2
Your board does appear to have a bios update related to cpu, N61PM507.BSS
http://www.biostar.com.tw/app/en/mb/introduction.php?S_ID=315#dl