You can only control the brush type (Ie, cylinder, cube, arch, etc.) when the brush tool is selected. If it isn't, then the selection box will be come grey in color, meaning you can't use it.
To alter how the brush is clipped, drag the line in the direction you want. If it wasn't exactly how you wanted it, then you can click and drag on the 2 white squares at either end of the clipping line to move them into place. Then just hit enter!