Roof leaks in a steel roof:
Where at all possible locate the leak by viewing its arrival in the attic space under the roof proper. This will give you an accurate realization as to where the water is penetrating inwards. If there is paper fitted under the roof it may be traveling quite some distance before showing up where it has down below in the house.
If there is no roof space to crawl into then measure off an outside window to where the leak is coming into the house. Or use an existing chimney as your guide point to get into the general area of the leak.
If the roof is rusted out and the hole is obvious then proceed to measure up for a replacement of this damaged area in long run if possible or if a very old roof at least the same sized sheets as are used now. If the leak is not obvious look at each nail and look for any sign of a gap into which water will simply track and flow under.
Take a silicone gun with a good flowing silicone in it and simply with the tip cut back about a 45 degree angle proceed to swirl in figure eights around the outside of any suspect nails and up and over the top to complete each seal like this.
More often than not take special care to go over the fixings on the flat side of the ridge or other flashings. As with expansion and contraction holes can open up to let in a surprising amount of water where it used to be perfectly well sealed before this damage occurred.
If the leak is down the outside wall check the spouting is not overflowing down in behind the fascia board. Or if there is a center or parapet roof gutter take some rags and after cleaning off the joints. Inspect for cracking and if necessary reseal with a swirling of silicone over the top.