How can I access this field (_mktoReferrer or Referrer URL) to add it to an alert email? I have a pop-up form set up and want to be able to track what page the visitor came from. I can see it in the activity log details.

[Megan_Raftery]

[Josh_Hill13]

you should be able to see this as

Original Referrer

or

{{lead.Original Referrer}}

Alternative, if you cannot, you can use Visited Web Page + Constraint and add it to a custom String field.