I'm actually not sure what can cause this issue, maybe it's the high resolution of the mac display. It's working fine on my windows machine. Have you any other computer to try? or a virtual maschine? The tool itself is abandoned from the original author, so there won't be any new updates, sorry.
Alternatively you can use the new google cloud signer at your Google Play Developer Console.
that's strange. I'm using JRE 8 as well, so it should work. The tool does open, but it has problems then (so it seems in account of your screenshot). What OS do you use? Looks like MacOS? I haven't tried it on my Mac yet, only Windows.
Thanks for your help with this. I re-downloaded JDK8 and JRE and Apk-signer, thinking that I have initially downloaded the wrong version. After re-installing this, the signer doesn't start anymore - It just shows an icon of JarRsrcLoader. I can't Open With Java, as I can't find that in the available applications with which to open.
I've attached various screen shots.
There is no ".exe" file. It's a Java application. You just need to download the zip from the main page and unzip it. Then you will get the "apk-signer-...jar" file. Double click this to run with Java. If Java is not the default application to open this file, do a right mouse click on it and select Open with => Java. See image posted above.
If you still have problems, please provide a screenshot of your steps.