I have copied my saved file to a new folder but don't know how to load it from the main menu. Please could you explain this to me step by step.
I may not understand your problem right but I will try to make it a little clearer what you need to do.
When Carl recommends that you copy the files, this is so you'll have a backup in case you make a mistake. Once you've made a copy of the files you should just leave the copies as they are and go
back to the Saves folder where you found them; here:
In Vista, head to your Documents folder (C:\Users\USERNAME\Documents\Electronic Arts\The Sims 3\Saves
In Windows XP, the folder is found at C:\Documents and Settings\USERNAME\My Documents\Electronic Arts\The Sims 3\Saves
It's in that Saves folder you need to delete the .sims3 file and rename the backup, like it's described here:
Once you have safely copied the files, delete the main .sims3 file and leave the backup. Once the .sims3 file is gone, right click and rename the backup file to WHATEVERITWAS.sims3 (removing the extra .backup extension).
What you should end up with is a file called WHATEVERITWAS.sims3 in the Saves folder. It needs to be in that folder for the game to be able to find it.
Now when start up Sims 3 again the saved game should be where it usually is.