I noticed an .htaccess file in the screenshot of the MODX root directory you posted in the other thread.
Check the Friendly URLs (FURLS( System Setting (Go to System -> System Settings and type friendly in the search box at the upper right).
If the use FURLS setting is on, you need the .htaccess file.
If the use FURLS setting is off, you need to *not* have the .htaccess file (rename it).
I'd recommend the second option for now (FURLS off, no .htaccess). That will take FURLs out of the picture while you try to sort things out.
If that doesn't do it, try right-clicking on the site_start page and select "View" -- see what URL that sends you to and whether you see the page.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html