But the DS only has 4 MB of RAM, and DSLinux uses up a huge chunk of that so that by the time you're done you've only got a few hundred KB of free RAM for running programs. Try it on a small text file first.
Regardless, once you get inside and are presented with the flashing cursor, type "vi path/to/file.txt". Note that to access the root of your card it's /media/ and spaces must be preceded by a backslash for Linux to handle them properly. Therefore, to access a file in "(root)\Ebooks\Alice In Wonderland.txt" you would type:
Code:
vi /media/Ebooks/Alice\ In\ Wonderland.txt
Tab-completion makes this process much much easier. Start typing the first few letters of the file or folder name and hit Tab to have it automatically complete the name for you. Much faster, and it automatically adds the backslashes for you.
Once you've got it opened, hit the slash key and type the phrase you wish to find and hit Enter. To escape Vi, press Shift+; to create a colon, type q! and hit Enter to force it to close without saving any changes.
Vi is a bit complicated, I'll admit, but the page on the DSLinux wiki for Vi explains it better than I can. Use it if you get stuck.