The issue seems to be related to the JAVA version used to compile your application compared to the JAVA version used in NeoLoad.
NeoLoad 5.1 uses JRE 1.7.xx version. It seems that the JAVA version used to compile your application is 1.8. Could you confirm that with your developers?
NeoLoad will use JDK 1.8 in the next version. You could try to switch the JDK version in NeoLoad 5.1 from 1.7 to 1.8 but i cannot guarantee that NeoLoad will behave properly.
It's worth a try.
To do so, download the JDK 1.8.xx from Oracle website and replace the <jre> folder in NeoLoad installation folder by the one that you've just downloaded. Do not forget to rename it jre too.
Edit <install_dir>\conf\agent.properties and remove the option "-Xbootclasspath/p:lib/npn-boot-1.1.7.v20140316.jar"
Restart NeoLoad.
Try again to load your JAR's files.