I've been downloading various new food but some of them aren't showing up while I play the game. I have made sure that I'm trying to cook the food at the right time as well as having the correct cooking skill and appliances. I am using The Sims 2 with no EP. Some foods that won't show up for me:

http://www.modthesims2.com/showthread.php?t=210576
http://www.modthesims2.com/showthread.php?t=211595

Another note; most foods I downloaded from http://exnemsims.com/ do show up except for the banana waffles found at http://exnemsims.com/forum/showthread.php?t=1317

Sounds like you have an out of date food hack or something- test with a clean Sims2 folder:
http://www.modthesims2.com/article.php?t=139718

I made the clean Sims2 folder and added only one of the foods that don't work to the downloads folder. Unfortunately it still doesn't show up. What does it mean when the food hack is out of date?

Does it work if you make a clean Sims2 folder and don't add any custom content at all except the food? By food hack I was meaning that perhaps you had a hack relating to food that was not suitable for Pets.