you need to have a column in your cart to mark items as webinars or not.
for example sake, lets say we call this column "category". In the add to cart buttons for webinrars, set the value of this column to "webinars", in the add to cart buttons for other products use another value.
in the triggers for the webinar discounts, use the trigger:
"Number of items with a specific column value"
If the number of items where "category" has a value of "webinars" is "> " a value of "0"
AND:
Session "userLevel" is set
then for the calculation use "Based on items with a specific value"
Total of "TotalPrice" where "category" has a value of "webinar" "plus" "0"