I'm not familiar with the Wichita refuge; my bison experience comes from the Yellowstone area. That said, I would never describe bison as being particularly "active": they spend most of their time either resting or grazing, with the herd slowly moving on to uneaten patches of grass. If you want to see bison running around, mating season (on the Wichita refuge, June and July) will see bulls competing for mates.
The best time of day to look for bison is whenever you feel like it. Bison don't make any effort to hide, so you shouldn't have any difficulty spotting them if they're in the area.