The official website for the Lord of the Rings Online has important information for those who have received an open beta key from PC Gamer. It is quoted below:

Greetings to all PC Gamer readers who have obtained their Open Beta key in the May edition of PC Gamer! This key will allow you access to The Lord of the Rings Online™: Shadows of Angmar™ World Tour Open Beta beginning April 6. To find out how to activate your key and download the game please visit http://www.lotro.com/openbeta.

Click here for more information.