Originally from Whyalla, South Australia, Didak made his South Australian National Football League (SANFL) senior debut for Port Adelaide Football Club aged 17 years. He was the captain of the Under 18 Australian International rules football in the junior International Rules Series against Ireland.
Didak was drafted by the Collingwood Football Club with the 3rd pick in the 2000 AFL Draft, and in 2001 he made his AFL debut against the Kangaroos at the Telstra Dome in Round 7; gathering 10 possessions and kicking one goal. He played five games in total in 2001.
In 2002, Didak earned an AFL Rising Star nomination. Didak was an extremely talented small forward and was also very capable in the midfield. He was known for his accurate goal kicking and clutch ability which often saw him likened to club legend Peter Daicos. In 2005, Didak's season was disrupted by injuries and problems. He had knee surgery during the pre-season, setting him up for his first game on ANZAC Day against Essendon. His injury had an effect upon his performances. Having had minor issues since the mid-year break, he was suspended in Round 14 for two matches and after returning from that, he injured an ankle. When he returned in the next week, he lasted only 20 minutes, before suddenly fainting on the ground with an irregular heart beat.
In 2006 Didak was awarded the Copeland Trophy as the best and fairest Collingwood player for 2006.
Didak was delisted at the end of the 2013 season after 13 seasons.
Didak played one game for the Glenorchy Football Club in the TSL in 2014.
In late June 2007, it was revealed that Didak had met with Christopher Hudson, the self-confessed shooter in the 2007 Melbourne CBD shootings days before the shootings took place. According to police, Didak left the Spearmint Rhino with Hudson, where Hudson then allegedly fired random shots before travelling to the Hells Angels' East County Chapter headquarters in Campbellfield. Police believe that Didak was later dropped off near Southbank after the shooting incidents about 6.00am.
On 3 August 2008, Didak was a passenger in a car being driven by intoxicated teammate Heath Shaw which collided with another parked car. Both men initially denied that Didak was in any way involved. However, witness accounts to the incident identified him and, on 4 August 2008, both men were fined and suspended for the remainder of the season over the deception.
On 6 September 2012, Alan became a first time dad to daughter Indiana Willow Didak with his long term partner Jacinta Jellett.