Glowstone Dust is obtained by destroying Glowstone Blocks, which are only found naturally in the Nether. It can be mined with anything, though it is mined quickest by a sword (however using a sword will deduct two uses from it). Glowstone Dust can also be obtained by putting a Glowstone Block into a Macerator, which will yield 4 Glowstone Dust. This is a better way to obtain Glowstone Dust as opposed to simply breaking a Glowstone block, as breaking a Glowstone Block does not guarantee the return of four Glowstone Dust.
Glowstone Dust can also be crafted with Gold Dust and Redstone, but only if it is enabled in the ic2.cfg file (it is a disabled recipe by default).