Thank you for getting back to me.
I found the solution and wanted to share in case any one else comes across this issue.

use the following code resolves the issue of dividing by zero OR dividing zero by zero.

=IIF(Fields!Presentations.Value > 0, Format(DateAdd("s", (Fields!TalkTimeSeconds.Value/
IIF(Fields!Presentations.Value > 0, Fields!Presentations.Value,1)),"00:00:00"),"HH:mm:ss"), "00:00:00")

Thank you,
Cervello