The key for this action isn't mappable through the in-game UI. While in Workshop mode, pressing the 'F' key will rotate the object you're trying to build and place. If you're still experiencing this problem, and if 1.3 is still in beta, you can install the beta via Steam by following these instructions. Update: Fallout version 3.1-in beta as of this writing-resolves this problem (hallelujah).
