Your responses tell me you didn't follow the installation guide correctly.
The v0.3.28.3 file you downloaded from the link IS the JAR. You don't extract it.
GoFile:
View attachment 2332739
Mega:
View attachment 2332736
MixDrop:
View attachment 2332738
UploadHaven:
View attachment 2332740
If you are running on a Windows x64 computer, you first download from THAT link which is a compressed folder with the EXE and also in there is the monthly build of the JAR file (v0.3.28.0). Extract the compressed folder to your computer. Double Click the EXE file to play.
If you want to use the current weekly update JAR of v0.3.28.3, then you download it from the appropriate link - make sure it it named correctly and replace the JAR that is already in that game folder.
Voila!