I just got a DSi and i love it, and i tried to convert the files to aac. when i put the Sd card in the ds, it couldn't recognize the format. please help.

You need to have the format into AAC Format otherwise known as mp4 format like iTunes format.

Right-click songs in iTunes and say "create AAC version" or whatever.

Page 44 of your DSi manual says:

The following file formats can be played with Nintendo DSi Sound:
AAC files with .m4a, .mp4, or .3gp filename extensions
Bitrate: 16kbps - 320kbps
Sampling rate: 32kHz - 48kHz

Make absolutely sure your files were saved under one of those three extensions and converted into one of the acceptable bit/sampling rates and try again. You didn't mention which program you're using to convert your files, but there's a tutorial for doing it in iTunes in this post here.