I just bought a SB600 for my Nikon D3000 but I can't find the correct settings?

The flash doesn't come up all the time. I tried different settings but I wasn't happy with them.
Can you please help me with this.

Certain settings on your mode dial disable the flash. Look at the chart on page 176 of your manual. It looks like you can only use the SB600 with P, A, S, or M. Can that be true? The pop-up works in several other modes…

Regardless, yes, there ARE modes where the SB600 is disabled.

The SB600 when mounted on the hotshoe of the camera will disable the pop-up flash, which usually pops up automatically when the camera is on Auto or any Scene mode.

You need to have the camera in P, A, S or M to use it. The SB600 can be set to TTL (this is an Auto flash mode, where pre-flashes are sent out to measure the exposure, then the flash is automatically fired at the 'correct' power output to light the the subject. TTL BL is a similar mode where backlighting is taken into account.

Like most Auto modes, TTL often gets it wrong, usually because the camera is metering in the wrong place (matrix metering can fool it easily). Spot metering is often best for portrait work.

You're best bet is to learn how to use the flash in manual (ie YOU decide the power output, based on how you want to light the subject).

Try this for starters - set the flash to manual (think it's pressing the Zoom and - buttons to change mode, though I don't have mine in front of me at the mo'), then mount it on the camera. Set the flash power to 1/8 (reduce/increase it by pressing the -/+ buttons). Put the camera in manual, ISO 200 & f8. Flash exposure only cares about aperture, shutter speed controls how much of the ambient light is in the scene.

Try the shutter speed at say, 1/60 (you'll get the 'room' light in a bit) and a 1/200 (max speed you can use with the flash) which will kill the ambient light. If your subject is too dark, increase the aperture or the flash power, if too light, do the opposite.

When you're confident about flash, I suggest you get it off camera where you can get really creative! A set of RF602 radio triggers (which will fire you flash off camera) costs about £27. Off camera flash will allow you to change directionality of the light and modify it to make it softer or harder.

Have a look at this if you really want to know about off camera flash;
http://strobist.blogspot.com/2006/03/lighting-101.html