What's the order of MLs to get credit?
To qualify to receive credit for the MLs you participate in, you first talk to the Arbiter near the first port after boating in to ToA.
Then you must solo one of Krojer's Champions off the coast of Volcanus Haven. After completing each ML, you return to the Arbiter for credit. Also, after completing ML 1, Arbiter will offer you a choice of two paths to follow. Be sure you read the skills gained in both "tracks" you are offered before deciding.
The MLs are basically a set of trials to be completed individually, in groups, and in battlegroups. There will be a series of ten trials in each master level and the only requirement is that you complete the final trial last. For example, you can do ML 1.5, then 1.2, but 1.10 can be done for credit after you've completed the previous nine steps.
Can MLs be done out of order?
Yes! You can complete the MLs in any order (almost). For example, say you had the chance to join an ML 2 event before you completed ML 1. Once you have completed ML 1 AND gotten sufficient ML experience, you will receive credit for both MLs.
But there's a catch: In order to enter the necropolis, where ML 5 takes place, you must have completed ML 1. To enter the dungeon for ML 7, you must have completed ML 3; to enter the dungeon where ML 9 takes place, you must have completed ML 8.
MLs are certain to make your character more effective in PvE and RvR.