Flight crew of C750 descended prematurely when cleared for the GPS-D RNAV approach at SDL.

Narrative

While approaching destination SDL from FLG VOR; ATC issued an approach clearance. Proceed direct WILUP. Cleared GPS-D to SDL. We were approximately at POURS (IAF) when clearance was received. The SIC set the altitude selector to 4;600 FT and said we could start down. He held up the chart with his finger indicating the selected altitude. While descending Approach Control issued an altitude alert and we immediately climbed. I asked for the VOR approach and Approach said I'll give you a vector for the visual. He said Scottsdale Tower was reporting good weather there. He leveled us at 6;000 FT and gave us a vector toward the airport. Approximately a minute later; he started our descent and we saw the field at approximately 5;500 FT MSL and about 7-8 miles away. We completed an uneventful landing. I should not have accepted the GPS-D approach. I falsely assumed we would be in VMC prior to arriving at WILUP and would commence a visual approach. It is against SOP to use a GPS approach that is not in our FMS database. I did not thoroughly review the approach. I should have seen that the 4;600 FT altitude that the SIC put in the Altitude Selector did not begin until after crossing TAGOC waypoint. I normally conduct my flights to a high professional standard. I am thankful that this event did not have a bad conclusion. I will conduct myself in a highly professional manner in all future flights.