I don’t know if the Secret Service is the right agency to report this to. They have had a pretty good time the past 8 years. Only a couple of the parties have been exposed. They are probably pretty happy with the status quo and the election of Trump might upset it.
Yes, the secret service is the right law enforcement agency to report this threat to. Donald Trump is under secret service protection.
The secret service has already talked with Beck once already when he publicly fantasized about stabbing Trump repeatedly on a debate stage. Though when questioned by Dallas based secret service agents, he claimed it was his radio co-host Stu, not Trump, whom he would stab repeatedly.