Can i use my old MacBook charger on my new Mac? I have a 2011 15" MBP, that uses a 85W power supply. 
My old 2008 MBP 15" also has a 85W power supply, but I cannot get it to work. Shouldnt it? Its the same watt power, and the adapter fits.
The way Apple power adapters are designed is that you can use any charger on any laptop that has the same, or lower, power rating. For example, you can safely use an 85W power adapter (for MacBook Pro) on a MacBook Air (which requires only 45W) or MacBook (which requires 65W).

However this does not work the other way around, you cannot use a MacBook Air (45W) adapter to power a MacBook Pro.

A 2008 MacBook Pro adapter rated at 85W should power a 2011 MacBook Pro with no problem. If this is not the case, then the adapter is faulty.