This is actually very clearly covered by this document published by the Department of State, which is linked from the page on the Visa Waiver Program:

• Conference, meeting, trade show, or business event attendee: Will receive no salary or income from a U.S based company/entity. For scientific, educational, professional or business purposes.

• Lecturer or speaker: No salary or income from a U.S. based company/entity, other than expenses incidental to the visit. If honorarium will be received, activities can last no longer than nine days at any single institution or organization; payment must be offered by an institution or organization described in INA 212(q); honorarium is for services conducted for the benefit of the institution or entity; and visa applicant will not have accepted such payment or expenses from more than five institutions or organizations over the last six months.

As you are going there unpaid, this is clearly within the activities allowed by a Visitor (B) Visa, and thus for the VWP.

This is actually very clearly covered by this document published by the Department of State, which is linked from the page on the Visa Waiver Program:

• Conference, meeting, trade show, or business event attendee: Will receive no salary or income from a U.S based company/entity. For scientific, educational, professional or business purposes.

• Lecturer or speaker: No salary or income from a U.S. based company/entity, other than expenses incidental to the visit. If honorarium will be received, activities can last no longer than nine days at any single institution or organization; payment must be offered by an institution or organization described in INA 212(q); honorarium is for services conducted for the benefit of the institution or entity; and visa applicant will not have accepted such payment or expenses from more than five institutions or organizations over the last six months.

As you are going there unpaid, this is clearly within the activities allowed by a Visitor (B) Visa, and thus for the VWP.