The aircraft has 3 ILS receivers, but only ILS L and R in the CDUs. How does the ILS C is put into action?
It depends on which page you are viewing. There is the NAV RADIO page, and the ALTN NAV RADIO page.
In standard operation (i.e., nothing has failed, more on that in a bit), there are no separate L and R ILS entry lines on the NAV RADIO page (unlike for VOR and ADF).
A single entry, on either CDU, tunes all 3 receivers to that frequency. If FMC 1 and 2 failed, the alternate nav radio page (ALTN NAV RADIO) can be used. And this is where the third CDU comes in.
If both FMCs fail, ILS receivers can be tuned on the CDU ALTN NAV RADIO page. The left ILS receiver is tuned with the left CDU, the center receiver is tuned with the center CDU, and the right receiver is tuned with the right CDU.
Source: Boeing 747-400 FCOM 11.20.6 (emphasis mine)
Here's the location of the center CDU:
Note: Normal operation is based on auto tuning, say unlike a 737NG where the frequency is manually selected left and right. With the approach selected, and within range, the plane auto tunes the frequency. The crew can then confirm the identifier on their displays.