Just drag an .sseq file to sseq2midi and it will create a MIDI file. Likewise, you can drag a MIDI file to midi2sseq to create an sseq file for use in a DS ROM.
Incompatibility Notes Edit
According to the main site, Jump/Call/Loop events in the original SSEQ files are lost when they are converted into MIDI files.
Furthermore, some MIDI events (i.e. Aftertouch) cannot be implemented in SSEQ file.