See the Raster Extensions topic.
SlopeHeight(s, g) (was: Slope)
SlopeHeight(s, x, y)
SlopeHeight(s, x, y, w)
Returns the slope of s at the specified location. The location is either the centroid of a geom or a pair of XY coordinates. Passing the w parameter computes slope over a window of specified size (1 for 3x3, 2 for 5x5, etc). Surface Tools only.
So you need either (a) an edition that includes Surface Tools or (b) another edition plus the Surface Tools extension (or the Option Pack).
To see whether a current Manifold 8 installation includes Surface Tools, see Help > About. If it does, it says so.
Everything else you need to know is on the Products page. Some previous version 8 editions are no longer offered (pricing was simplified).
(None of this applies to version 9.)