A couple of other things to check, if the problem isn't that you are typing %:
1. In a DOS window, type:
echo %path%
and make sure that you see the path to the "bin" subdirectory of your
Java home directory as one of the entries.
2. Type the command:
java -version
to ensure that the JVM can be found.
3. If you then still cannot successfully get the command:
javac filename.java
to work, it's possible that you accidentally downloaded the Java Runtime Environment (JRE) vs. the Java developers kit; the JRE does *not* come with javac.exe.