the maps and mods i cant help you with as i have trouble with mods myself, but the snapshot is pretty simple.
1. On the toolbar open up your documents tab and click open in finder.
2. From there you will click on the Library tab.
3. In the Library folder open application support
4. Scroll down until you find your MineCraft folder.
5. In your minecraft folder open up the bin folder.
6. Right click once on minecraft.jar and click duplicate.
7. drag the snapshot file you downloaded and replace the original minecraft.jar
8. Open minecraft and enjoy
Also just to let you know if you go to mojang's website they have a pre-release of the new update. They also put out a server client. so let your favorite server owner know to update it : )
1. On the toolbar open up your documents tab and click open in finder.
2. From there you will click on the Library tab.
3. In the Library folder open application support
4. Scroll down until you find your MineCraft folder.
5. In your minecraft folder open up the bin folder.
6. Right click once on minecraft.jar and click duplicate.
7. drag the snapshot file you downloaded and replace the original minecraft.jar
8. Open minecraft and enjoy
Also just to let you know if you go to mojang's website they have a pre-release of the new update. They also put out a server client. so let your favorite server owner know to update it : )