Firstly, that's the 10.1.5 update.
There are a couple of reasons why it would be grayed out. One is if you are already running 10.1.5 (check this by selecting About This Mac in the apple menu) or that you haven't 'authenticated' the installer by clicking the little padlock and entering an administrator password.
I've done this a lot of times since moving to OS X. I'll find myself wondering why a dialogue is grayed out, and it will take a moment to notice the little padlock in the corner is 'latched'.
- iMac G5 1.8GHZ 17" | SuperDrive | 160GB | 512MB | Airport Extreme | Bluetooth Keyboard & Mouse | Wacom Intuos II
- Pentax *ist DL - JVC MiniDV Camcorder - Airport Express - iPod Nano 1gb white