IFR Corporate Jet with ZMP enroute to EAU; requested ILS 22; ATC's clearance was misinterpreted; reporter initiated approach procedure before clearance for the approach was received.

Narrative

On IFR flight plan; destination was EAU. I asked Minneapolis Center for vectors to the ILS 22 approach. Center responded that he could not give vectors for that approach; so he instructed me to maintain 5000 FT and proceed direct to MAGGS. He then instructed me to report the procedure turn inbound. Since MAGGS is an IAF for the approach; and since he told me to report the procedure turn inbound; I flew the procedure from MAGGS outbound and the procedure turn as if I had been cleared for the approach. I did not realize that he did not clear me for the approach. So after passing MAGGS outbound I descended to 2900 FT; as depicted on the approach plate. Inbound on the procedure turn Minneapolis Center called me and asked for my altitude. I responded that I was just turning inbound on the procedure turn; and I was at 2900 FT. He said I should be at 5000 FT; as he had not cleared me for the approach. He then said I was cleared for the approach; and I completed the approach and landed successfully. When he told me to report the procedure turn inbound; I inferred; (incorrectly); that I had been cleared for the approach. I had never before been told to execute a portion of an approach procedure without having been cleared for the approach. In the future I will be more careful to listen for those 'cleared for the approach' words before changing altitude from that assigned by ATC.