DTXMania files
You can find DTX Files at GDMania web site.
DTX File format is explained at DTXMania Developper Web Site ( Japanese/English )
RockBand files
How To create RockBand Files at Creators Rockband Web Site
**** TO USE ROCKBAND FILES YOU HAVE TO BUY ROCKBAND (tm) 1 , 2 or 3 games, depending which files you want to use ***
**** IF YOU USE DLC FILES , YOU MUST BUY DLC at RockBand Store => http://www.rockband.com/store ***
*** THIS WEB SITE DOES'NT INCLUDE ANY ROCKBAND FILES ***
So when you have the .mid file, you must create a Ogg Vorbis Background music soundtrack that will play with the Midi File.
If the MIDI file is named MySong.mid the Ogg file must be MySong.ogg. If no Ogg file is found, there will be no background song.
Also you must install Ogg Vorbis Codec, see the Download Page.