i'm using v6, and as far as i know curves are not available - but you can assign linear interpolation. the interaction is roughly as follows:

in the effects channel, double click on the red line representing the current value - a marker should show up. now double click somewhere else to get a second marker. you can now drag the markers independently and ableton will interpolate a line between them, resulting in a smooth transition. double click elsewhere to create another vertex, rinse, repeat.