Here's a link to the adult version of Insimenator. You must be 18+ to register and view the content on the site, but it has virtually everything you need to make your sims appear more natural in game.
These links gives you some options for the men:
Insimadult...Marvine's Ulterior Motive AllenABQ's Hairy SkinsThis one gives you options for the ladies:
Warlokk's Tower Unfortunately, mostly every site I know of has the rar files for download as opposed to the zips. I'm not sure if you just need a better utility, but here's a link to the 7-zip which opens zips and rars and many other compressed files and it's free.
7-ZipAs far as making the changes to your existing sims, there are many tutorials using SimPe to make the changes to them, or you could just find a default version of a skintone that you like and drop it into your downloads folder thereby changing all of the Maxis nudes to something more realistic. The effect can easily be reversed if you delete the default package from your downloads folder.
I'll get a couple of links for you to tutorials which are available.
These tutorials use SimPe which is currently being updated for the Season's EP and it is advised that if you have Seasons installed, you wait for the updated version before using the program for editing sims. Some people still use it with no ill effects, however, I leave that to you're discretion.
Insimenator...Tutorial-Giving your in-game sims a permanent make-over.
Mod The Sims 2...Default skin replacment tutorial Mod The Sims 2...Creating default skin replacement - simplified method~~~~~~~
LOL I just read a duplicate thread of this on Insimadult. Guess you knew all along.
