how can I add mosfet model in ADS?

Which model you want to use ? Select the model which you would in "Devices-MOS" palette and define your parameters of your MOS device in "Properties" of model block.
(All kind of models may not be in the palette in ADS.. )

As BigBoss said you can add parameters manually, alternatively some of vendors supply designkit for ADS you can install them from the main console.

I want to use of BSIM3.3 in ADS2008 and add new libary to ADS
I think that have to define new path

plzzzzzzzzzzzzz help meeeeeeeeeeeeee

BSIM3.3 is looks like spice library . download them and use File --> Import --> More option --> PSPICE

Regards,
ALPER USLU

thanks for replybut I want to know that can I use the same library that use in HSPICE?or I need apply another library?

ADS2008 may not recognize BSIM3.3 model..Check first this
Otherwise use later version like ADS2009U1..

I know that this way but I want to add 0.18um technology to ADS

In this case you have to extract the model parameters for certain WxL of MOS and then you'll fill them up..
Another way to do this, request their PDK from 0.18um foundry which you intend to use.
If you ask me "how to extract model parameters", I say it's a very serious modelling engineering and it can not be done without very expensive measurement equipment and huge of knowledge..

I did this way but it had error in simulation ( File --> Import --> More option --> PSPICE )

plzzzz help meee

File import won't work for your component.It's a importing utility for a SPICE based network. As I said before, you should define your MOS parameter in Model block and then you assign this model in your MOS component properties dialog.

ok it is ok
now I want to detect how I can realize transistors in which regoin?