The crew of a Skynorth Air Ltd Beech 100 was on a re-positioning flight from Thompson, MB, to Island Lake. On arrival in the Island Lake area, the crew commenced an instrument approach to runway 12. During the approach, the aircraft descended and the crew carried out a missed approach. During the missed approach, the aircraft struck a tree. The crew continued the missed approach and returned for a landing at Island Lake without further incident. No injuries were reported. The aircraft sustained damage to the wing leading edge and underside, and to its landing gear doors. FINDINGS AS TO CAUSES AND CONTRIBUTING FACTORS: 1. The crew conducted an Area Navigation (RNAV) approach for which they were not trained, with an aircraft that was not properly equipped nor approved for such purpose. 2. The aircraft descended 300 feet below the minimum descent altitude (MDA) as a result of a number of lapses, errors and adaptations which, when combined, resulted in the mismanaged approach. 3. The aural warning on the aircraft’s altitude alerter had been silenced prior to the approach, which precluded it from alerting the crew when the aircraft descended below minimum descent altitude. 4. The SkyNorth standard operating procedures for conducting a non-precision approach were not followed, which resulted in the aircraft descending below the minimum descent altitude. During the ensuing missed approach, the aircraft struck trees.