Using EOF 1.42 or Lower
To mark a section of a song as Guitar Solo, just open the midi file in EOF and click in front of the solo part.
Then go to Beat > Events and type "Solo On" to mark it as the beginning.
Finally, go to the end of the solo and do the same only call it "Solo Off".
Using EOF 1.5 or Higher
Highlight the notes going into the solo by clicking the beginning note of the solo and shift clicking the end note of the solo. Click on Notes- Solo- Mark.