Pixi, in order for anyone to be able to help you, you need to post the error log. The message you posted told you where the error log was. Find it. Attach it to your post. Maybe then someone will be able to help you.
Ah, sorry, I've attached it to this post. And I tried to get two of the Sims engaged to test if the arch works that way, but it keeps wanting me to propose "move in" first. I don't want them to move in together--that's part of the reason I got the arch in the first place, and I've read that you can get them engaged without having them move in together, but it doesn't seem to show up. When I had two Sims dine out, at one point there was the option of "surprise engagement", but that doesn't seem to be there anymore.
The Sim I want to get hitched is in love with three different people, and I was hoping to get him married to all three of them. One is a pre-created Sim (Weldon Dallas I think his name is), and the other two are characters I created (who just happen to be married to each other... yeah, I dunno how it happened either). I don't know if that helps or not... if you need any other information, just ask. I'd really like to get this working, thank you.
EDIT: Stupid n00b question... what's InSim?