How to attach wheels to the base of slotted-angle rack?
You'll use one of the variants with the plate on top, and you'll use three of the four slots for carriage bolts. The carriage bolts should have square lugs that fit the size of the slots. 

If you find that the corner bolt doesn't have good bearing, or the hole wouldn't be well-positioned with respect to the ends of the metal components, you many need to first install corner plates of heavy steel (1/4" or better). These could be anchored to the bars with four bolts positioned outside the area occupied by the caster plates. 

The variants with the pins would require metal bores to properly support them against that load. You'd have to bolt metal blocks into your frame. It's not a good strategy.