To look around hold RMB (right mouse button) and drag your mouse to the side that you want to look to.
In order to move horizontally use the WSAD keys:
W - forward (also mouse wheel can be used)
S - backward (also mouse wheel can be used)
A - left
D - right
Vertical movement is achieved by the Q and E keys:
Q - down
E - up
The whole movement is relative to the camera.
Movement speed can be adjusted by pressing keys from 1 (slow) to 9 (fast). The same effect can be achieved by using the “Camera speed” slider on the Editor quick settings: