With the client upgrade that L1.5 has made, the support for the really old mac client had to be removed due to the Korean servers of Lineage no longer supporting macs. To work around this problem you’ll need to install windows on your mac via one of many options. As there are many guides for this already out there, I will simply post some links that give instructions for doing this. From there you can simply install the game as per the regular windows instructions.
There are two main ways to install windows on your mac. As a bootable operating system using bootcamp or as a virtual machine (that is often somewhat laggy or buggy). Here are some links of guides to do this.
Boot Camp Guides
Virtual Machine Guides
There are many other guides or virtual machine softwares out there, players are welcome to post other guides or links to sites to help with this as a reply to this post.
For the most stable gameplay its recommended to play via the bootcamp method as your mac is formated and treated like a PC, however I will not blame anyone for disliking the idea of booting their wonderful mac into a OS like windows.