Are you upgrading or doing a fresh install?

in the case of upgrading try the following:

- Upload a new core (/concrete folder) to your site, this can be the exact same version you are running now.
- add /tools/upgrade?force=1 or /index.php/tools/upgrade?force=1 behind your domain name (in the browser you use, not in the site.php) to run an "upgrade".

if you upload the same /concrete as the version you are running it still "upgrades" to that version.
Doing this results in fixing any missing files and database pieces.

In the case of a fresh install, remove all files/folders. download a new concrete5 version and upload it.

Go to /heplers/form.php and add Site before the class names like class blah would be class Siteblah