How do you get to secret lots in the Sims 2 free time?

To get to a secret lot, first your Sim/s must have a Membership card to the secret lot. They must have at least 5 enthusiasm points in that hobby to receive it.

Then, to go to the lot they must organise transport, and when you get to choose the destination, select Hobbies and any lots they can visit will be displayed.

Hope this helps.