You have to open it with winrar.
You download the mod and open it with a archiving program, Win-rar is what I use but there is also 7-zip, then you have to find your .minecraft folder. You can find it by searching %appdata% in your start menu. Open the .minecraft folder then open the bin folder. Inside the bin folder, there is the .jar. Open it with a archiving program. Look at the mod. If there is a Readme file, open it and follow what it says to do. Also, there is usually the instructions on the page that you found the mod. If there isn't a Readme folder, then open the mod with the archiving program and copy all of the .classes into the .jar or your .minecraft folder. Caution: If you get a black screen when opening minecraft after installing the mod, then it is because you didn't delete the META-INF folder in your minecraft jar. Hope this was helpful. Ask me if there is any other problems. -Aiya
When you open the .jar, you're then in the root, meaning the top 'level' which contains all the other folders and files.
download minecraft forge, open it up and keep forge as is and hit ok, then find a mod, download the jar, and locate your minecraft folder go to mods folder, open that up and drag the mod jar into the folder and enjoy your mod. P.S. go to edit profile on minecraft launcher page and go to: use version, then scroll all the way to the bottom and click on the forge thing and then hit play and enjoy your mod.
download it then chuck its contents in a mods folder in minecraft .jar
Install bukkit
If you are using window's, then use the Run program, and type in %appdata% and then run that, and it will bring you to a file containing the .minecraft folder.
It does you just need to go into youthen go to roaming then to .Minecraft then click bin and it should be in there. What about for Mac?
firstly download mod then got to start menu and type run then type %appdata% then enter and look for .minecraft folder open it and go to bin then open the mnecraft .jar folder with winrar then copy and paste the too many items into mincraft.jar delete meta.inf then run minecraft. and enjoy
If your dragging the file on top of the jar then thats because the jar is a file and not a folder. If you drag a file on top of another file your say that you want to run file B with file A. Get a zip Archive software like 7zip (google it) If your using a archive software and this is still happening the only think i can think to tell you is open the archive.
firstly download mod then got to start menu and type run then type %appdata% then enter and look for .minecraft folder open it and go to bin then open the mnecraft .jar folder with winrar then copy and paste the too many items into mincraft.jar delete meta.inf then run minecraft. and enjoy
you need to download the jar on minecraft wiki or somewhere else. And then go into your home folder-library-application support-minecraft- and then go into bin and replace the jar with the one you downloaded. MERRY CHRISTMAS
you can either force update your minecraft via the login page or you can delete the entire bin folder