m_pp is a private member of the Client class (see zone/client.h), so if you were trying to manipulate m_pp.platinum within a method/function of the Client class, it should work.
If you are trying to do this from within a method from another class, then you would need a pointer to a client object, then you could use
Code:
Client::TakeMoneyFromPP(uint64 copper, bool updateclient=false);
If you need further pointers, I would need to know where you are trying to insert this code into the existing source.