I'm converting something from TS4 but I don't know how to get the TS2 nude meshes. I tried exporting a pair of underwear from bodyshop, but the package file generated didn't include the mesh. Can someone please give me instructions? Googling didn't help.

In SimPE, you can extract the meshes with the PJSE tool. Choose "new file", then the PJSE tool (Tools -> PJSE -> Body Mesh Tool -> Extracting stage -> write the name of the mesh).

They're called "--bodynaked", "--topnaked" and "--bottomnaked" with the age/gender prefix (ef/em/af/am/tf/tm/cu - pu/bu only has the body mesh, YA uses adult meshes), so for instance the fullbody AF mesh would be "afbodynaked".

You can extract meshes from recolors with the PJSE tool, too (Extracting stage -> "Browse..."-> find the recolor you want to extract from)

The Unimesh tutorial here has pictures and a more detailed explanation:
http://simswiki.info/wiki.php?title..._a_Mesh_Package (it also shows how to link them to a texture, if you need that).

If you need the face and heads too, those are called "--face" and "--hairbald" (same prefixes, elders and YA use adult ones).

Extract the GMDC if you want to use textures on them, as the OBJs have some issues with the UVs (The faces can cause some errors as GMDC if you import them to Milkshape, but they have major UV issues as OBJs).

To get at the Maxis nude meshes, I use a set of Invisible Clothes by Drachenfels that I downloaded long ago from out adult sister site SexySims. Drachenfels shared them mainly for people making skins, but I use the mainly when making skin-tight clothing, like underwear and swimwear. I'm not posting a link because officially it's adult content, but it's easy enough to find on the site, or I can send you a PM with the link if you like. Drachenfels's set includes separate top and bottom nude meshes, which can be hard to find otherwise.