Units being transported in a Chimera enter and exit the vehicle via the access ramp on the back of the hull. Because of this, models may only embark if they are within 2" of the ramp, rather than within 2" of the vehicle itself. By the same token, models that disembark must be placed back on the table within 2" of the access ramp.
Access Ramp
Source: Codex: Armageddon, p. 29