LTspice Adding Downloaded Models

LTspice comes with lots of nifty circuit components, but there will be many times that you want to simulate a part that isn't in the default libraries. LTspice makes this very easy for the vast majority of models that you will get from vendors.

We'll use the National LF411 opamp as an example. Go here and download LF411.MOD to your schematic directory to make the part available only to that schematic, or place it in C:\Program Files\LTC\SwCADIII\lib\sub to make it available to any schematic in the future. Open it up in a text file and look for a line that says
.SUBCKT LF411/NS 1 2 99 50 28
This line tells SPICE that the name of this object is LF411/NS, and also describes the what each pin means. We'll need this name to reference the part later on. Back in LTspice, press 's' to insert a SPICE directive. In the textbox type
.include LF411.MOD
This tells SPICE to treat that file as a library that contains model definitions. Then insert the part called opamp2. We'll use this as the symbol for the model we just imported. Right click on the opamp and change the Value to LF411/NS (Remember I told you that name would be important?). There could just as well be several different models within a single library file, each with a unique model name.

And that's all there is to using models!
Credit : http://ltspicelabs.blogspot.com/