On trial since June 2007, Nsengimana is accused of genocide and crimes against humanity committed in Nyanza, southern Rwanda. He has pleaded not guilty.
The protected witness alleged that the meeting was held at the college premises and had spotted the accused standing next to a gendarme.
The meeting was attended by about 30 people and Tutsis were mentioned as enemies to be eliminated "at a convenient time".
"Two days after the meeting at the college, when we awoke in the morning, we noted that houses [belonging to Tutsis] were set on fire in Nyanza", the witness added.
Some of the participants at the meeting were employees of the College of Christ the King, he alleged before the UN court.
The prosecutor alleges that Hormisdas led "death squad" made up of employees of the college.
Nsengimana is one of the four catholic priests indicted by the ICTR.
Athanase Seromba, a former vicar in Nyange, in western Rwanda, was sentenced to 15 years jail last year and has appealed against the decision.
Emmanuel Rukundo, a former military chaplain is currently on trial before the ICTR.
Wenceslas Munyeshyaka, a former vicar at the parish of the Holy Family in Kigali, will be tried in France.
© Hirondelle News Agency