first you choose a residential lot
build your building/furnish it
then you enter changelotzoning apartmentbase

there are 2 things (that I can think of) that might interfere with the sectionals
connecting the way they should.  First, make sure you're not trying to place the
sectional pieces with the snapobjectstogrid false cheat activated.

The other thing is an old sectional global mod by shaklin- it works well with
Phelana's custom sectional (at meshkiste)- but borks every other sectional in
game- do a search for shaklin_selectional_sofa_global in your downloads folder.

you can't rent a furnished room without cheats

the info in this link should help
http://www.sims2wiki.info/wiki.php?title=Tutorials:Building_an_Apartment#Renting_Furnished

are you not able to find the mailbox in your game?

It's in buy>misc (the 3 dots icon)>all (infinity symbol icon)
scroll through that catalog and you should be able to find it.

Painting custom paintings is a feature that started with Pets.
If you don't have the Pets expansion pack (or higher, not sure if it carries
forward) you'll likely not get the option to paint a custom painting from
your easel.

You're right, Hexameter did make curved walls at SMB/PC Sims-
but I don't think they made it to the graveyard.

you need to have the shader file in your downloads folder as well- download it from TSR (it's a free download)
http://www.thesimsresource.com/artists/Murano/downloads/free/sims2/objects/

it may very well be the case that the glasses you downloaded override
the maxis mesh, try removing the ones you downloaded and see if
Cassandra's glasses show up.

you won't find them in the catalog.  the pic is attached to the side of the camera, think polaroids- only sideways- you should see a white edge sticking out.  
you do have to go into buy mode to grab the pic- much like you would do
for a painting on an easel.  from there you can hang it on a wall.
fair warning though, the pic can be a little tricky to grab.

you're welcome, I hope it works out for you.

you're welcome, I hope it works out for ya.

It sounds like you're missing terrain paints.

You should probably go back to where you downloaded the lots from and
see if there are terrain paints listed for you to download.

Once you've downloaded the proper terrain paints, you'll have to delete the lots you already installed in your neighborhood as well as deleting them from your lots and houses bin and re-install them.

Alternatively, you could also try this cheat- it doesn't remove the terrain paints, it just makes them invisible-
boolprop lotTerrainPaints [true/false]

it appears that there are still some components of the radiance lighting mod
still installed in your game.  

It looks as though there are some people having the same issue, posting in the scriptorium thread.  Perhaps you should post your problem there as well-
http://modthesims2.com/showthread.php?t=192613&c=0&ht=0&page=14&pp=25#startcomments

There's a suggestion by Numenor -


Also, you can find a back up of the original Maxis lighting files in the
Gunmod's_Radiance_Light_System_2_3_beta thread at More Awesome Than You
http://www.moreawesomethanyou.com/smf/index.php?PHPSESSID=86b40f6e68de3acfaeed7658161298d1&topic=9811.0

nope- those particular hair styles are exclusive to the Sims 2 Store.  
They weren't released with any expansion packs or stuff packs.

the hair on the box isn't included in free time- ridiculous, I know.

what other hair do you think you're missing?