Instructions
Windows
Locate the "bin" folder for Minecraft. In Windows XP, it's at "C:\document and settings\[UserName]Application Data\.minecraft\bin." In Windows 7, it's at "C:\Users\[UserName]AppData\Roaming\.minecraft\bin"
Open the "minecraft.jar" file using a file decompression tool.
Copy the contents of your texture pack folder into the "minecraft.jar" compressed file folder.
Close the "minecraft.jar" folder and run the game
Mac OS X
Click on the "Macintosh HD" icon on your desktop and browse to the "Applications" folder in the "Places" menu on the left-hand side.
Click on the gear-shaped icon in the top menu bar of the "Applications" window and click "Create New Folder."
Name the new folder "Minecraft." Really, it can be called anything, but naming it "Minecraft" makes it easier to keep things organized.
Move "Minecraft.app" from the Applications folder into the "Minecraft" folder.
Copy the contents of your texture pack folder into the "Minecraft" folder and run the game.