If your Mac has a Core Duo CPU, you can't.
If it has a Core 2 Duo or better CPU and at least 2GB of RAM, try purchasing Mountain Lion from the Mac App Store. If that's incompatible, phone the online Apple Store and order a download code for Lion.
If you can't or don't want to upgrade the computer's OS(10.7 and newer don't support Microsoft Office 2004 or any other PowerPC software), buy an iWork DVD from a source such as Amazon or eBay.
I'm having the same problem, I have already downloaded the free software but when i open it it tells me that i need 10.7
Model Name: MacBook Pro
Model Identifier: MacBookPro5,2
Processor Name: Intel Core 2 Duo
Processor Speed: 2.8 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 6 MB
Memory: 4 GB
Bus Speed: 1.07 GHz
Boot ROM Version: MBP52.008E.B05
SMC Version (system): 1.42f4