Are you using DVD Architect Studio or DVD Architect Pro? (If you're not sure, it will say in the title bar.) DVD Architect Studio does not do alternate angles, so not much more to say if that's the one you have.
For DVD Architect Pro, add the main copy of your movie to the project and activate its timeline. In older versions, there will be a button above the timeline for inserting a video track. In newer ones, you have to click the plus-sign button to reveal an insert menu...use the one whose hover text is Insert Video Track. After DVDA adds the second video track, drag and drop your second angle on it. Alternatively, select the new track, hit the Insert Video Event button (a couple down from the plus-sign), and locate your other video. That should do it.