The manual advises us to create a folder and put there all minibb files, i.e. in http://mywebsite.com/forums . Can I place the files in root that is in http://mywebsite.com/. Forum is the only content in my website. Will there be any security problem or will the paid addons stop working? Thanks.

Can't see why there should be a problem - just make sure you set
$main_url and $pathToFiles in setup_options.php correctly.
All the best,
mb

I have changed $main_url to http://mywebsite.com

$PathToFiles appear as './'

Do I need to change it? How do I change it? Thanks.
Excuse my ignorance.

I figured out. Thanks.

Read about difference between $main_url $pathToFiles.