The easiest and fastest way to find out ILS frequencies for FS
are (as Brettbrett21 said) on the map page of your FS
. Simply click the map icon (looks like a target, circle with a + going through it). This will open up th map page, simply scroll along your depicted flight path until you reach your desired airport. Double click on the airport, this will open up another window giving you all the airports relevant information, such as runways and frequencies. Find the runway you will be using and write down the frequency and heading. Close the map page and you'll be back in your cockpit (opening the map page will pause your flight). Enter the frequency into you NAV1 radio and make sure it's not on stand by. Enter the heading into your COURSE selector on your autopilot panel. Once you're vectoring for the runway, change the GPS/NAV selector to NAV. Once ATC clears you for the ILS, switch on the APP selector on your autopilot panel. That's it.
As CO777ER mentioned, you have other options such as FSNAV, these are all extra programs that one can install into FS
to make things more realistic and easier. However, I find alot of these programs are hit and miss and involve alot of trial and error. As a private pilot myself, I like to simplify things. What I have done, is I spent an afternoon writing down all the airport information for the airports I use most frequently, put them into sections (North America, Europe, Asia, etc.), print them out and put them in a binder. That way I have everything I need, easy at hand and easy to read and I don't have to fiddle around in the cockpit or on the screen. Also, if I fly into newer airports, I'll write down the information and once a month, I'll update my binder.