I'm not completely clear on what you want. I thought you wanted your language strings to show in the MODX Manager (and I got the tag format wrong above -- I was thinking of how you load lexicons in code).
If you just want to show your custom language strings in the front end, creating your own file (in the right place) and using the correct language tags will do it.
If you create a lexicon file called mystrings.inc.php and put it in the core/lexicon/en/ directory with stuff like this in it:
$_lang['my_custom_string'] = 'What I want to appear';
This tag should show the string from that file:
<p>
My Custom String: [[%my_custom_string?
&language=`en`
&namespace=`core`
&topic=`mystrings`
]]
</p>
You could also put the file somewhere else and define a namespace (other than core) in System -> Namespaces.
For example, if the file were: core/components/mydirectory/lexicon/en/mystrings.inc.php, the namespace path would be: {core_path}components/mydirectory/ and the tag would be:
<p>
My Custom String: [[%my_custom_string?
&language=`en`
&namespace=`mydirectory`
&topic=`mystrings`
]]
</p>