The DS cart is not memory mapped, anything you want from it you have to copy into memory. Most games will use the onboard file read commands that came as part of the DS SDK but you are not restricted to it if you want to go manual ( http://problemkaputt.de/gbatek.htm#dscartridgeprotocol
If you are going in for assembly/something like a binary then you might also consider using the overlay system. The DS sets aside a chunk of memory (and any number of addresses within it) that binary fragments can be loaded into.
Even with the overlays though finding space can be more easily said than done in a lot of DS games. Doable, especially if you aggressively manage it, but still annoying.
If you are editing the first DS game then it is harder but for the second you might also want to have a look at any wifi error codes that might exist, they are not so useful these days (the DS wifi was shut off, though there are hacked servers up if you want that option) and take up the kind of space you can fit a lot of instructions in.