Chris Hadfield and Scott Parazynski work on 'Big Arm'
Canadian astronaut Chris Hadfield and US astronaut Scott Parazynski spent more than seven hours unfolding the arm from the shuttle bay and wiring it up.
"It looks real good," said Hadfield, the first Canadian to walk in space.
The astronauts sped through their chores, hooking up power, data and video cables for the new space station arm and then installing a radio antenna on the orbiting laboratory.
The 58-foot Canadian-built arm is critical to the continued construction of the space station.
The $1bn arm will act as a high-tech construction crane, able to walk end-over-end and use both ends as 'hands' to add pieces to the station.
It is the most advanced robotic device ever sent into space.
It is also so long that it had to be folded to fit inside Endeavour's payload bay.
And it is so heavy - 3,618lb of aluminium, steel and graphite epoxy - that it could not support its own weight on Earth.
Fresh fruit and electrical supplies
On Saturday Endeavour docked with the station, bringing Alpha's crew of three their first visitors since they arrived in March for a 4 1/2-month stay.
But the two crews will not meet face to face until Monday, as the hatch between Endeavour and the station remained closed to maintain different cabin pressures.
They did, however, exchange items through an outer station compartment, including mail, fresh fruit and electrical cords.
During their 11-day mission, the shuttle astronauts will also attach an Italian-built cargo carrier Raffaello to the station to start unloading 10,000 pounds (4,500 kilograms) of supplies
It will be the second of three Italian cargo carriers to be launched to the space station. Leonardo hauled up supplies last month.
The space station, under construction since 1998, is scheduled to be completed in 2006.