There are answers to many of your troubleshooting questions on the Makey Makey site.  Scroll down and click on Makey Makey to get to them:

Makey Makey FAQs

ADST and Coding ADST Home Page ADST Curriculum Makey Makey What is it? Getting Started What's in the Kit? Lesson Ideas Troubleshooting Tips

ADST and Coding ADST Home Page ADST Curriculum Makey Makey What is it? Getting Started What's in the Kit? Lesson Ideas Troubleshooting Tips