I want to make this cube taller. I've selected a loop around the cube (in the middle and at the bottom) but can't seem to figure out what tool gets me there. Extrude and slide loop don't do it.

forgot. here is the image

Well you could scale it on the Z axis. If you want to stretch it you can select the top faces and use the proportional editing.

If you want to keep the loops in the same proportions, then scaling on the z-axis will do it (ctrl-s, z, [enter number or move mouse]).

If you just want to move the top up, then select all the vertices/edges/faces from the middle to the top (or any number of levels from the top down) and pull it on the z-axis (g, z, [enter number or move mouse]). You can then add more edge loops into the elongated space if you need them.

Or, select all the top faces (only) then extrude (e, z, [enter number or move mouse])

If you want to scale the cube as a whole and make it taller click on "A" to select all then "s" and then "z" to scale it up the z-axis and drag your mouse or you can enter a numeric value by just typing it from the numpad

thanks guys. problem sorted.