1. Reformat your microSD card. Use FAT16 if you can.
2. Download the latest firmware (v1.09) which you can find
here.
3. Unzip the firmware. Place TTMENU.DAT and the TTMENU folder (with its contents) at the root of your microSD card. "Root" means the top level (not in a folder) of your microSD card.
4. Create a folder (name it "games" or whatever you want) at the root of your microSD card.
5. Copy one or two .nds files into the "games" folder of your microSD card.
6. Remove the microSD card from your PC using the "safely remove hardware" utility.
7. Put the DSTT into your Nintendo DS.
8. Put the microSD card into the DSTT.
9. Turn on the Nintendo DS. You should see the .nds files in a list. Select one to play.
-Bri