There's an icon on the panel (with what looks like an "A"), next to other icons, like the GPS, etc. You click the icon and then the stack will pop open and you can input the ILS frequency into NAV1.
... if you are referring to the Piper Meridian, it may not have the icons as they appear in default FS2K2 planes. You may need to look at the manual that's probably included in the software.