First, use the transporter which has a fly sitting on top of it (third from the left). As a mix of Rufus and a fly you'll land on the other side. Click the transporter on the right. You'll return to Doc, now use the third transporter from the left (it is currently empty). Use the upper transporter. When you've made it to the location with Doc, teleport to the rebels using the third transporter on the left. Use the upper transporter once again - there is a frog inside.
As Rufus mixed with a frog click on the fly found in the first transporter from the left side. Travel to the rebels using the transporter you're currently in. Return to Doc using the bottom transporter, and then travel back to the rebels using the first transporter on the left. Use the ladder.