At first check the locations of the file java.policy:
sudo find / -name "*java.policy"
There may be several files:
/Applications/Processing.app/Contents/PlugIns/jdk1.7.0_51.jdk/Contents/Home/jre/lib/security/java.policy /Library/Java/JavaVirtualMachines/jdk1.7.0_40.jdk/Contents/Home/jre/lib/security/java.policy /Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home/jre/lib/security/java.policy /Library/Java/JavaVirtualMachines/jdk1.7.0_51.jdk/Contents/Home/jre/lib/security/java.policy /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/lib/security/java.policy
Usually you can ignore the files in /Application. Edit the rest, e.g.:
sudo nano /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/lib/security/java.policy
Add the following line within the block “grant { .. }” and save:
permission java.awt.AWTPermission "accessClipboard";
Repeat this with the other files until you can use the clipboard.