The 27-year-old was released from the final year of his contract with Melbourne Storm.
"We know we are going to get better as the tournament goes on, " said the Melbourne Storm player.
King joined the club from Australian side Melbourne Storm in 2007 and has become a popular figure at the club.
The reigning Super League champions will take on Australian NRL title holders Melbourne Storm at Elland Road on Sunday, 28 February.
And that is what happened in 2010 when Australian Rugby League team the Melbourne Storm were caught systematically breaking the competition's salary cap.
Manly qualified for the world title match by defeating last year's World Club Challenge finalists Melbourne Storm 40-0 in the NRL Grand Final.
Tadulala first came to prominence representing Fiji in the 2000 Rugby League World Cup before joining Melbourne Storm where he stayed for three seasons.
The 28-year-old Melbourne Storm back rower was due to arrive earlier this month but is still awaiting clearance to link up with his new team-mates.
The 23-year-old not only captained Australia to a stunning 58-0 victory over New Zealand in October, he also skippered Melbourne Storm to the NRL crown and helped Queensland win the State of Origin.
Leeds have won Super League's last three Grand Finals and bid for their third World Club Challenge at Elland Road on 28 February when they face Australian National Rugby league champions Melbourne Storm.
As well as Harlequins scrum-half Orr and Warrington full-back Richie Mathers, wingers Nick Youngquest (Crusaders) and Willie Isa (Melbourne Storm), Cas have also signed three forwards, St Helens duo Jake Emmitt and Nick Fozzard and Huddersfield's Martin Aspinwall.
The 29-year-old Australian, who moved to England from Melbourne Storm in 2007, says it was the experience of playing cricket with his five brothers and his sister as a child in Casino, New South Wales, which helped him develop such a simple idea.
BBC: Warrington's Matt King is the inventor of the 'Crackit'
If Leeds manage to beat Manly in the World Club Challenge it would make them the first team in the 33-year history of the match to be crowned world champions in consecutive years after they defeated Melbourne Storm 11-4 at Elland Road last year.
For sure, the boundaries are being blurred, as both codes seek to encroach on each other's territory. (Last year, for instance, the Sydney Swans won last year's AFL grand final, while the Melbourne Storm took the honours in rugby league, which would have been unthinkable twenty years ago).
Crocker, who has signed a three-year deal with Hull, is continuing to train in Melbourne with his former Storm team-mates.
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