you need to buy the game for the CD Key and create an account. Then you go to
www.lineage2.com and download the Test server client.
Though I want to warn you. most Korean MMORPGs nowadays are just clones of L2. Lineage 2 is a memory hog, it'll make the fairest of computers cry. Mainly because instead of getting rid of the data for an area you've been to hours ago, it just stagnates on your RAM.
Also, the game is a griiiiiiiiind and a half.