multiple comparisons

Dear "R" users,
I need a professional help:

i am a relatively new "R" user and I am just writing my diploma tesis where I have to conduct some multiple comparison.
I am searching a method which include interaction between fixed factors. The following is my model: lmer(Leaf~water*region+(1|pop)+(1|pop:sib)+(1|block)+(1|block:pool),data=datx,method="ML")
I want to compare: leaf dependent from water * region.

I would prefer to make a comparision according to tukey or student newman keuls if this is possible. 
I am looking forward to your reply, sincerely Petra:-)