@wingshigh: In FC3 you only need to open the radiomenue, press F5 and select the proper airfield and announce the ready for taxi/takeoff or startup.
In Bs or A10 you would have to setup the correct airfieldfrequencies.
If you ask for taxi, the tower will tell you which runway to choose. (he will tell you a two digitsnumber which are the first two digits of the course, so 24 would be hdg 240 and 01 would be 10) But there won't be any advices which will guide you to the runway so make yourself familiar with the airfieldlayout and know how to reach the right side of the runway.
For landing you just have to call inbound and the tower will talk you to the ILS turning point.
@lumpypilot: If there is a hole in the runway they will divert you to the other base, right. I would suggest you check per map your distance to the diversionbase. There is a rulerlike tool, which provides you heading and range to the field. Alternately you can select the airfield through the radiomenue and call the inbound. Again they will guide you to their ILS turningpoint. (where you would have to turn into the runway)
Another idea would be to check preflight where the field is located, fly (following the waypoints) to your normal airfield and then head to the diversionbase by just remembering where it was. You should be able to find it that way and if you are close you can just cycle through the airports in ILS-Navmode and take the one which has the closest range (should be somewhere around 0-2nm) That way you will be able to get the ILS Steerbars.