PMA and EGF will preferentially go through ERK1/2. I use TNFa to activate all three ERK1/2,JNK and p38 and see a nice difference compared to EGF or PMA.
50-100 nM PMA. You may want to perform a time course initially to determine the optimal time. Also do a basal point. Note that as you are using cultured cells, serum contains growth factors etc and so you should serum-starve the cells. Your lysis buffer should contain phosphatase inhibitors.