Nikon D90 - How do you turn on Manual Flash Settings? (Built In Flash)?

I bought myself a D90 to upgrade from the D3100, but in the menu's there's no way to switch on manual flash, and I don't have a flash gun (I'll invest in one soon, but for now I have to use the built in flash)

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I'm using a Macro Filter, with the D3100, I could switch the flash to Manual, allowing me to adjust the amount of light it produces. There was also TTL, which (I assume) is Automatic. On the D90, I can't use the Macro Filter as I can't switch the flash off TTL.

Are you asking how to open the flash manually? There should be a button on the front, left hand side of the viewfinder. You can manually control the flashes output by using the +/- button next to that button.

Look on page 4, 70 and 91 in your user manual