Suppose your Pi has ip address p.p.p.p and the server subnet for your OpenVPN is v.v.v.0/24.
Then what you need is:
configure set protocols static route v.v.v.0/24 next-hop p.p.p.p commit save exit
Edit:
I see you did this while I was writing this. I run OpenVPN on a PogoPlug under Arch Linux and this kind of route on my ERL. It works for me.
You didn't do that MASQUERADE from the post above did you?