It is not clear what you want/need. Do you want to add a transistor manufacturers library (Designkit) or do you want to define a transistor from the transistor model parameters (netlist/text based)?
The procedure to install and use the designkit in ADS 2011 is described in the online help in topic "Working with Upgraded PDKs in ADS 2011"
If you use ADS 2011, you will need a designkit that was upgraded for ADS 2011. Earlier designkits (2009 and before) can not be used with ADS 2011.