The sso_secret should be the same as the one you added to MODX
The sso_url should be the full URL where your [[!discourse? &loginPage=`54`]] snippet resides.
Also set the "logout redirect" setting (under Users) should contain the full URL of your MODX logout snippet.
The next time you load Discourse (while logged into MODX) you'll see your MODX account details showing up in Discourse.
&loginPage - This is the resource id of your MODX
login page. Unauthorized users will be redirected to this page. If the
parameter is not included with the snippet the value of the login_page
system setting will be used instead. This param will also accept a URL
in case the page you want to redirect to is outside of MODX.