Run Pad Designer in the “PCB Editor Utilities” folder. Create a new padstack with File->New.
Set the units to Millimeters, since this is what is specified in the drawing.
Switch to the layers tab and select the “Single Layer Mode” check box Set the Begin Layer and Soldermask to rectangles with a width and height of 1 and 1.4 millimeters, respectively. This may not be ideal but will work fine for my purposes.
Update: Looking at a TI datasheet it seems that under-sizing the soldermask by ~0.07mm is ideal.
Save the Padstack and open “Orcad PCB Editor” to test it out. If you’ve used a non-default directory to store your padstacks you will have to add the directory to your user preferences (Setup->User Preferences ->Paths->Library->padpath and psmpath).
Create a new package: File -> New -> Package Symbol. Select Layout->Pins and then open the previously created padstack from the options menu on the right.