If you have the beta patch installed correctly, and it is in the Arma 2 Operation Arrowhead/Expansion/beta folder.. this is what I do for every beta patch that comes out. Copy the arma2oa.exe from your beta folder. Then, go back to the main Arma 2 Operation Arrowhead fold with the other .exe's. Rename the existent arma2oa.exe file in this fold "backup". Paste the beta arma2oa.exe app into this folder.
Log in to steam and go to the launch options for Arma. Paste "-beta=Exansion\beta;Expansion\beta\Expansion;CA;@DayZ" and "-mod=@DayZ" into the launch options without quotations. Add any other launch options you see fit (i.e. world=empty and -nosplash).. And you should be set!