The curvature is determined by a couple of properties on the EdgeDrawer owned by the GraphDrawer owned by the NodeXLControl. Do this before drawing the graph:
EdgeDrawer edgeDrawer = nodeXLControl1.GraphDrawer.EdgeDrawer;
edgeDrawer.CurveStyle = EdgeCurveStyle.Bezier;
edgeDrawer.BezierDisplacementFactor = 0.2;
As described in the NodeXLApi.chm help file, the EdgeDrawer.BezierDisplacementFactor property "is the distance of the edge's Bezier control point from the midpoint of a straight line connecting the edge's endpoints, expressed as a multiple of the straight
line's length." Larger values result in greater curvature. The NodeXL Excel Template uses values between 0.1 (low) and 1.2 (very high).