Hi, for someone to help you, you need to provide more details, it would help if you could supply the following:

What add-on are you trying to install?

What version of Concrete5 are you running?

Is there a link to the site you are working on?

This is a start and may assist someone trouble shooting the problem for you.

Also, if it is a free or bought package from this site, then you should be able to get help from the add-on's author, on the add-on home page look on the right side to find the "Support" link.

Regards