Click "save as", name the file minecraft-report.bat
Underneath where you put the file name there will be a pull down menu named file type, select the file type as All Files, and click save.
Run the bat file that you just created. Try to duplicate the crash. Upon crashing, an error report will be created, named "log.txt". Congratulations, you have just created an error report.
Open the log.txt file
Once you have obtained the crash report, right click on it, select all and press Enter to copy, then paste it to the proper mod thread for assistance.
Go to Macintosh HD/Applications/Utilities and open Console.app
Run minecraft and when it crashes the java log will be added to the console, which you can then select and copy
This method is similar to Windows' method 2 in that you can use a separate file to launch Minecraft.
Create a new text file and open it
Make it a plain text file (In TextEdit, use ⇧ Shift+⌘ Command+T)
Copy and paste the following code: cd ~/Library/Application\ Support/minecraft/bin; java -Xms512M -Xmx1024M -Xincgc -cp "minecraft.jar:jinput.jar:lwjgl.jar:lwjgl_util.jar" -Dorg.lwjgl.librarypath="$(pwd)/natives" -Dnet.java.games.input.librarypath="$(pwd)/natives" net.minecraft.client.Minecraft
Save the file with any name you like, but uncheck both "Hide extension" and "If no extension is prvided, use '.txt:'.". Use extension ".command"
Run the file (which will open a Terminal window), and the next time Minecraft crashes, the log will be spit out into Terminal
Open a terminal (on distributions such as Ubuntu/Linux Mint, this is done with Ctrl + Alt + T)
Run java -Xms512m -Xmx1024m -jar minecraft.jar and watch the console output. You can select and copy it after making the game crash.