Yes. Your license includes the rights to use the script on a website (commercial, personal, client), or intranet site project.
No. The extended license is still limited to a single end product. You can refer to Details and Licensing help pages to get more information.
You can use mightySlider in your ThemeForest item if you purchase one extended license for that purpose. Meaning 1 Extended License for every single ThemeForest item you upload (1 ThemeForest Item Upload = 1 Extended License, 2 ThemeForest Items Upload = 2 Extended Licenses etc.).
Feel free to make any modifications you need. Also feel free to blog about it and show your friends, but please do not redistribute the script with your changes.
There are a few things you can try to get this working.
Be sure that you've included the script after you've included jQuery, as it is most certainly dependant upon that.
Make sure you don't have multiple versions of jquery in your page. Look at the source view in your browser to see what jquery scripts you are loading.
For example:
<script src="crazy_path/jquery.js" type="text/javascript"></script> ... <script src="crazy_path/jquery.min.js" type="text/javascript"></script> ... <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.8/jquery.min.js"></script>
jquery.js and jquery.min.js are the same script. Delete both and keep only one.
This is likely a doctype issue. This plugin requires a valid doctype and rendering in quirks mode is not supported. Make sure you are using the full doctype declaration to insure rendering in standards mode.
This abbreviated doctype renders the document in quirks mode for Internet Explorer:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
The doctype with URI renders in standards mode for all browsers:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
I recommend using the HTML5 doctype.
This could be because mightyslider.css is not included or the images are not uploaded. It also happens when your own CSS is conflicting with mightyslider.css. Make sure you target elements in your CSS without setting things global, code like li { display: inline; } for example is bad practice when you use third party code like mightySlider. When your CSS is properly coded and all the required files are uploaded and included you should have no problem implementing mightySlider.