If you have installed the latest version of MPLAB,
the MPASM files should be located in the C:\program files\microchip\mpasm suite folder.

Sometimes the Find Automatically button doesn't find the correct folder.

In MCS, go to the View | Compile and Program Options | Assembler Tab

Click the Find Manually ... button and point it to
C:\program files\microchip\mpasm suite

Then check the Use MPASM checkbox.