Locate your java home directory by executing in Termal
This will return the full path to your java home directory on your Mac. Change into that directory and then you will be able to run the java keytool from there.
Here is a log of what I ran in Terminal on my Mac:
$ /usr/libexec/java_home /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home $ cd /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home $ keytool