1. Open you .minecraft folder

Windows XP / other - C: / Documents and Settings / username / Application Data / .minecraft 
Windows Vista / 7 - C: / Users / username / AppData / Roaming / .minecraft 

2. Backup up your worlds and minecraft.jar 
3. Open the file minecraft.jar in the bin folder using WinRAR or 7zip. 
4. Drag all of the files from the Jar folder in the Terraria 3d v1.2.zip
5. Drag all of the modloader and audio mod files from there separte zip files
6. Delete the META-INF folder
7. Go back to your .minecraft folder and go to the resources folder
8. Drag the Mod folder from the resources folder in the Terraria 3d v1.2.zip into the resources folder in the .minecraft folder
9. Open minecraft and your done