There are two lenses that I would recommend, but first I recommend that you experiment with the lens that you currently have. It is a good lens but somewhat "slow/dark" for concerts (at least those with low light that I'm thinking about). For concerts that have very strong lighted performers, it might do OK. By slow, I mean that the maximum lens opening is small and that restricts the amount of light that the lens lets pass to the sensor.
Nikon makes two lenses that are considerably faster/larger maximum opening. One is the 35mm f/1.8 AFS DX lens another is the 50mm f/1.4 AFS lens. The latter is considerably more expensive but has a longer reach; a reach very nearly the same as the 55mm telephoto position on your current lens. The reason that I recommend you experiment with the current lens is to see at what focal lengths you feel the image size is appropriate for your seating location and subject size (e.g. what magnification do you need).
The 35 and 50mm lenses recommended above are AFS lenses. That means the focusing motor is in the lens. Nikon has several other lenses with these same focal lengths but they are AFD lenses and will not fully integrate on your D5000. Your D5000 body needs lenses with the focusing motor in the lens to work optimally. The 85mm lenses that Nikon makes are other very popular short concert distance lenses, but they are AFD lenses and don't integrate fully with your D5000. These allow for longer "reach" than your current 18-55mm lens (the 85mm has a longer focal length than the 55mm telephoto end of your zoom and therefore presents a larger magnification over the zoom).
I feel some experimentation is useful to fully answer your question so that you get the maximum benefit from any new equipment.
My daughter uses the same kit that you now have and gets wonderful pictures of her family and their activities. That camera is really packed with capability and the lens is sharp and versatile. Low light concert venues tax even the best and most expensive equipment, so getting the right thing on the first go is desirable.
Roger It is still ISO, aperture and shutter speed, right? "Nikonians membership - My most important photographic investment."