How do sims get engaged on the Sims 3?

Build up a relationship with the other sim, first 'propose going steady' when you click on 'friendly' on the other sim, If they accept click 'propose marriage'. Hope it helps! However, make sure they are really close, 'cause if they aren't, the other sim will reject the offer, and they will not be as close any more.

Make sure it's a romantic relationship, ie: either the heart appears below the relationship bar or the boyfriend/girlfriend symbol, they have to be girlfriend and boyfriend or whatever, and I'm pretty sure that in the currrent interaction it has to say Extremely Irresistable in the top right corner. I'm having serious issues getting my sims engaged, but then I am trying to engage her to the Grim Reaper (who is already moved in and they are dating). <--lol