Do you have a 5-axis mill or router? If not, the software won't help. If you do, VisualMill Premium from Mecsoft will write the toolpaths for you. 4-axis machines usually have a rotary axis that holds the work parallel to the X (longest) axis. This is better for things that are basically cylindrical, or at least can fit within a cylinder, than for reliefs, though. Try Deskproto for continuous 4-axis toolpaths for parts like that.
But practically speaking, especially if you're on a tight budget, carving the bulk of the relief in 3-axis mode and putting the undercuts in by hand would be the simplest solution.