In the root folder (above your site folder) you'll need an .htaccess file with the following code:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^manager$ /yourSubFolder/manager/ [R=301,L]
RewriteCond %{HTTP_HOST} ^yourSubFolder\.com$ [OR]
RewriteCond %{HTTP_HOST} ^www\yourRootFolder\.com$
RewriteCond %{REQUEST_URI} !^/yourSubFolder/
RewriteRule (.*) /yourSubFolder/$1
</IfModule>
I also have this in my .htaccess, you may or may not need it depending on your host and where the php.ini file is located.
<IfModule mod_suphp.c>
suPHP_ConfigPath /home/mySubFolder
<Files php.ini>
order allow,deny
deny from all
</Files>
</IfModule>
In the sub folder where your MODX site is installed you'll need another .htaccess file with this code:
RewriteEngine On
RewriteBase /yourSubFolder
This works well until you pass beyond the home page on the sub folder's site and then the sub folder shows up in the url. I haven't figured out how to fix that just yet.