This is the procedure to enable ILS in the MSFS 2020 B747-8.  (It also works for the B787 Dreamliner)

Click the NAV/RAD button on the FMC.  If you see "ILS: PLAN" part way down the left column, enter your ILS frequency (with the period (.) in the number), a slash, and the ILS course in degrees (no spaces) in the scratch pad at the bottom of the FMC screen, and click on the ILS line (L4) to enter it.  For example, Yokota AB, ILS RWY 36 is 109.70 MHz at 357 degrees.  So, you would enter 109.7/357.  

If you add a STAR to the flight plan, the FMC should automatically enter the correct ILS frequency and course into the NAV receiver.  However, this does not always happen. 

So, be sure to check the NAV/RAD page to verify that the FMC accepted your frequency and course.  And, if the Execute button is lit, click it.