Diadem Peak sits at the north end of the head of the Woolley Creek drainage just north of the Columbia Icefield. It shares this area with neighboring peaks Mount Woolley and Mount Cromwell. Two Main routes to the summit of Diadem Peak are accessed from this side of the mountain, the South Ridge (II) and the South East Face (II) each of which are moderate climbs. Another route, accessed from the north side of the mountain and much more difficult, is the North Face, Humble Horse (IV, 5.7 WI4).
Take Alberta, Canada's Icefields Parkway toward the Icefields Center. Park your vehicle at an unmarked but very large pullout adjacent to the Woolley Creek drainage and about 12 km north of the Icefields Center. Wade across the wide and braided Sunwapta River (one may consider using a pair of sneakers which may be left on the opposite bank and used for the return) and find a good but steep trail heading into the woods on the south side of Woolley Creek. Climb this shoulder to bypass a large, cascading waterfall; it eventually gains and follows an intermittent path at creek-level. Follow cairns west into talus and moraine eventually turning toward the north until several good, walled bivy sites are reached. About 3 or 4 hours hiking time.
I have not done the approach to the north side of the mountain and, futhermore, have no second hand knowledge of it other than that which is available in Sean Daugherty's Selected Alpine Climbs book. He says that parking is available at an interpretive pull-out 19km north of the Icefields Visitors Center. Ford the Sunwapta River and follow Diadem Creek along a morainal crest on the right. Surmount the valley's headwall by hiking around its north side, then contour above the headwall onto the glacier below Diadem's north face. Good luck.
This area is located in the Jasper/Banff National Parks and a daily park pass ($19.60 cdn. per group/vehicle)is required for travel in, and use of, their facilities; these may be purchased at the park entrance stations located on the highways. An annual pass may be purchased for the price of 7 daily passes; if you plan to be in the parks longer than a week it makes sense to purchase one of these. Additionally, backcoutry bivy permits are required for backcountry camping; these may be purchased at the Icefields Center (among other places) for $9.60 cdn. per person per night. Strong parties may find it well within reason to access and climb the Woolley Creek side of the mouintain in one 12 to 14 hour day.
See above for bivy sites in the Woolley Creek drainage. Camping may also be found at the Icefields Campground for about $16.00 per night.
Diadem Peak and its most popular routes are covered thoroughly in "The 11,000ers of the Canadian Rockies", by Bill Corbett, and in "Selected Alpine Climbs in the Canadian Rockies", by Sean Doughterty.