Cool_buddy,
Since you are new to Orcad, you probably have not used a .lib file to create a symbol file (.olb). The file that you downloaded (even though it is named a .mod file) is really a .lib file. Simply rename it to LMV794.lib.
Once it is named .lib, use the Model Editor (found under the Cadence-->AMS Simulator-->Simulator Accessories) to open the .lib file. Use the option Export to Capture part Library... and a generic symbol for the device will be created in the same directory as the .lib file. This will be the .olb file.
Next, Open your capture tool and create a new project. Set up your project and select the Place-->Part... Navigate to your directory where the .olb and .lib files are stored by clicking on the file icon:
Once you have found the .olb file and loaded it, it will appear in the part window. Click on it and the part will be ready to be placed in your schematic.
Before you run your simulation, be sure to add the .lib path to your sim profile, otherwise the device will not be found for simulation:
Select Library and add the path to your design.
That should be all you need to do to simulate the device. Of course, you will need to draw the rest of the circuit around the device as well...
Britt