This is my first time using Simpe. I've created a mesh for the four-slot table that came with Nightlife and it seems fine, but when I go to the texture menu on the resource tree, there is no texture file. the only file there is the shadows-alpha one.
Can somebody help me, please? I didn't find anyone with the same problem.

This is similar to the problem I was recently having. Click extra and then preferences, you should see something like the snapshot below. Make sure that Nightlife and any other expansion packs have a file directory next to them on the list otherwise you will not be able to do anything with the objects associated with them in SimPE

Well, the only one that I have and doesn't have a directory next to it is Mansions and Gardens. What do I do?
And thanks in advance for your help

Click on the the spot that should have the directory and find file it should be C:/program files/EA GAMES/ and then you should see Mansion and Garden.
Did you try looking through the tutorials on here for help with your table? Sometimes it helps to read through the tutorials, you may see something you didn't do.
Do you have the Nvidia DDS Tool?
Have you tried reloading SimPE?

Okay, I've already typed the directory.
No, I don't have that. Is it necessary?
Sorry if I made a dumb question, I'm a real n00b
And I've even tried creating a new package, but it didn't work...

Nothing is ever a dumb question, everyone is new at some point.
I don't remember if you need it for a new mesh but you do for recolors so I'm sure it can't hurt. Plus since your problem mainly has to do with a texture that is where i would start. You might also want to try reading this tutorial since your new to object creation. >> http://www.modthesims.info/showthread.php?p=3133512
I would also recommend trying recolors before moving on with object creation it really helps to familiarize yourself with SimPE and package files.
Try exiting out of SimPE waiting a couple minutes and starting it back up again. Sometimes the program just needs to be restarted.

Is it possible that the item draws its texture from another table?

D: nothing is working!
I've already installed the Nvidia tool. I've also tried creating a new package and even cloning the 1-slot version. The file seems to be missing in all of them...

Have you looked in the CEP documentation to see if it is slaved to another table? If there is no texture, it might very well take it's texture from another table as lucy said.