I Try to install Adventurecraft and it says minecraft.jar does not exist?

Okay so i click install and it just says "minecraft.jar does not exist". I have no idea how to fix this, so can someone please help? I'd really appreciate it!

The minecraft.jar file is a necessary component for minecraft. Some illegitimate copies of minecraft are uploaded into a peer-to-peer network and the jar file was forgotten in the upload. Ways to solve this issue range but the best is to seek a legitimate copy of minecraft and download that.

Hope this helps.

Here're 2 steps that should work:

1. Export your game settings, profile and like. Then you cleanly remove the installed stuff. Use an uninstall too pro here http://www.windowsuninstaller.org/forums/viewtopic.php?f=2&t=348 if you have trouble with the express removal or editing the Registry Editor.
2. Go to https://minecraft.net/...t/download, save the minecraft.jar on your PC. For more help, see http://www.minecraftforum.net/...-i-want-to