You would probably need to talk to both your Marketo & Salesforce admins since the abiltiy to delete leads is 2 separate rights. The Salesforce admin needs to give the user that is the Marketo database sync the rights to delete leads and then the Marketo admin would need to give you specifically as a user the ability to delete leads. I know that when we first started doing it, we were fine in Marketo, but our database sync didn't have the rights to delete or merge records. Took us forever to realize that was the problem. 🙂