You need to build the bridge to get the creatures to the other side. The bits the creature can walk along are build along the green line. Your bridge needs to be made out of triangles to be effective. Otherwise it will just break. Use the orange fixed anchor points to support your bridge.
Build the bridge and press T to test it. Press escape to go back to building it. Click on pieces to delete them. Click on anchor points and press delete to delete those.
Use the "reset ball" button to place the ball where it landed last.