Here is the skeleton for a queue that uses message passing.

Note that you do not pass a queue as an input to a queue object. It is already a queue!

(define (make-queue)
  (let ((front-ptr '())
        (rear-ptr '()))

    ; empty-queue? is written to align with the way front-ptr
    ; and rear-ptr were given, above
    (define (empty-queue?)
      (null? front-ptr))

    ; peek returns the datum at the front of the queue
    ; peek returns #f if the queue is empty
    (define (peek)
      (cond ((empty-queue?) (error "Empty queue.  :-("))
            (else (car front-ptr))))

    ; insert-queue! plays out differently depending on whether the queue
    ; is currently empty or not
    (define (insert-queue! datum)
      (let ((new-node (cons datum ())))
        (cond ((empty-queue?) 
               (set! front-ptr new-node)
               (set! rear-ptr new-node))
               (set-cdr! rear-ptr new-node)
               (set! rear-ptr new-node)))))

    ; delete-queue! has three possibilties:
    ; * empty queue
    ; * one element in queue
    ; * more than one element in queue
    (define (delete-queue!)
      (cond ((empty-queue?) (error "Empty queue.  :-("))
              ; store the datum at the head of the queue
              (let ((return-value (peek)))
                ; update the front pointer
                (set! front-ptr (cdr front-ptr))
                ; If there was only one thing in the queue, then the
                ; rear-ptr will need to be set to nil
                (if (null? front-ptr) (set! rear-ptr '()))
                ; Now return the element of the queue (or #f)

    (define (dispatch message)
      (cond ((eq? message 'insert-queue!) insert-queue!)
            ((eq? message 'delete-queue!) delete-queue!)
            ((eq? message 'peek) peek)
            ((eq? message 'empty?) empty-queue?)))

Here is a stack abstraction that is similar in approach:

(define (make-stack)
  (let ((top-ptr ()))
    (define (empty-stack?)
      (null? top-ptr))
    (define (peek)
      (cond ((empty-stack?) #f)
            (else (car top-ptr))))
    (define (push! datum)
      (let ((new-node (cons datum ())))
        (cond ((empty-stack?) (set-top-ptr! new-node))
              (else (set-cdr! new-node top-ptr)
                    (set-top-ptr! new-node)))))
    (define (pop!)
      (let ((return-value (peek)))
        (if return-value
            (set-top-ptr! (cdr top-ptr)))
    (define (set-top-ptr! node) (set! top-ptr node))
    (define (dispatch msg)
      (cond ((eq? msg 'push!) push!)
            ((eq? msg 'pop!) pop!)
            ((eq? msg 'peek) peek)
            ((eq? msg 'empty?) empty-stack?)))