If you have an issue that can’t be fixed after trying the steps on this page, feel free to contact the developer.
Proof Pad requires an up-to-date installation of Java 7 (or Java 6 on OS X). Make sure your version of Java is up-to-date. If it is, try reinstalling Java.
If you get a message about ACL2 terminating when you start up Proof Pad, then the version of ACL2 that comes with Proof Pad is not compatible with your system. One thing to try is installing a separate version of ACL2 from the ACL2 website. After that, point Proof Pad to your separate installation by going to Tools > OptionsProof Pad > Preferences…Edit > PreferencesPreferences, check “Use custom ACL2”, and direct Proof Pad to your custom ACL2 installation.
There’s an older Windows version of ACL2 here that’s been especially reliable. It’s pretty out of date, but should work fine as a last resort.