Ad addendum - when I went back to the game I could at least interact, but as soon as I clicked on the one person he already knew who was exercising, that sim got off the machine and the game froze - actually the whole system froze and even control-alt-delete wouldn't work. I will try the challenge one more time in Willow Creek but am getting very frustrated with it all.
Not sure if it is just Oasis Springs? Didn't have any issues with my sims visiting the gym in Willow Creek, but haven't tried the body builder aspiration before.
I was reading the story of the player who is working on achieving all the "aspirations"and envy her dedication. My sims keep getting distracted with ""living"! (Even using cheats, especially to buy things needed to get the next level.