I created a custom field in the Opportunity called Actual Amount. It represents the Sum of all the Opportunities' Actual quantities (also a custom field). I would like to create a Rollup Summary of the Actual Amount and display it in a custom object call Events__c. I can't do it out of the box because the Event__c doesn't display the Opportunity object when I try to create the rolloup summary field. Any help would be much appreciated.

I created a custom field in the Opportunity called Actual Amount. It represents the Sum of all the Opportunities' Actual quantities (also a custom field). I would like to create a Rollup Summary of the Actual Amount and display it in a custom object call Events__c. I can't do it out of the box because the Event__c doesn't display the Opportunity object when I try to create the rolloup summary field. Any help would be much appreciated.