At the time of writing this article, ModX v0.9.6 was used. I believe the procedure has altered somewhat since that version so please check the official documentation if in doubt.
I've been working with Modx recently, and it's a cool CMS system. One area that I initially struggled with is getting a local site working on a hosting server. So here are my observations on what to do, or try, to get it working for you:
1. Create the database on your server
2. Upload all the site files to your server
3. In 'manager/includes' folder, make sure the 'config.inc.php' file is amended to suit your server database details
4. In 'assets/cache' folder, delete the 2 (probably) 'siteCache.idx.php' and the 'sitePublishing.idx.php' files.
5. Login in to the manager.
6. Go to 'Tools / Configuration' and select the 'Interface & Features' tab.
7. There, amend the 'Resource path' value to reflect your servers site path to the 'asset' folder. Then save.
8. Once saved, click the 'File Manager' tab
9. There, amend the 'File Manager Path' to reflect your servers site path to the web folder. Then save.
Your website should now work.