I downloaded the newest version of the game client (2.1.16) for Mac, but whenever I log into the game, all the characters and land features appear as black squares, while the user interface, chat box and ground (grass, paths, water) appear as normal. It's playable, but I can't see what anything looks like. I tried looking around, but it doesn't seem like any one else has experienced this issue.
Any ideas on how to fix this?
EDIT: Here's a screenshot for clarity:

EDIT: And my system specs:
Processor: 2.1 GHz Intel Core 2 Duo
Memory: 2 GB 667 MHz DDR2 SDRAM
Graphics: Intel GMA X3100 144 MB
Software: Mac OS X Lion 10.7.5 (11G56)