Yahoo doesn't allow Pakistani publishers so I doubt they take Pakistani advertisers. I suggest directly contacting Pakistani webmasters for advertising. Why go through a foreign firm if you just want to advertise on Pakistani sites?
I have seen Pakistani ads on Yahoo several times but I guess you are far better with Google than Yahoo or even MSN. You can have your ad displayed on MSN Meesenger main window. That'll work good too.
Try google, much better, I`ve seen pakistani ads on MSN, not on yahoo though, but i dont use it alot so would know.
Go for GOOGLE Adwords. YPN is just limited to US alone. If you want to target Pakistan, then why don't you go for local offline media.