Things should be visible at that detail. The process FT uses is: evaluate the surface, fill basins, find flow amounts, subtract flow amounts (with specified exponent and results clipped to 0) from the offset channel. The next screen refresh will recalculate the world with the changes.

It's possible that you're using a large blur and/or a large Amount, which can result in large areas of the higher flow (effectively farther along the rivers) being forced to 0 and so having no visible effect. What settings are you using? Try an amount of 1, flow exponent of 0.3, effect blend of 1 and blur of 0. If those settings don't result in visible channels in areas not at sea level then nothing will.

Wilbur is a good tool for learning the incise flow settings. It has faster setup and is a little bit easier to fiddle with.