To update your cheat database, grab the file that ends in "_USRCHEAT.DAT.zip" from http://cheats.gbatemp.net/Temp/ then unzip the archive and put the USERCHEAT.DAT file into the TTMENU folder of DSTT(replacing the one that's already there).
To enable cheats for an individual game, select the game and press the plus button on the touchscreen (or press SELECT) to be taken to the game options menu. Select the option to turn cheats on. Then select the cheats tab (or press right) to access the list of cheats. Once finished save your selection.
You can turn on and off cheats from within a game that's running using the following button combinations:
cheats off: L+R+START+DOWN
cheats on: L+R+START+UP