You need to insert your content into the DOM and call MathJax to render it -- MathJax needs the DOM to properly render your content.
MathJax uses custom script tags to store math sources in the page. For asciimath the script type is <script type="math/asciimath"> math content </script>
. See the documentation on the MathJax processing model for more details.
After you insert your content into the DOM, you need call MathJax. One way is MathJax.Hub.Queue(["Typeset",MathJax.Hub]);
which will render all new content in the entire page (but not rerender everything).
You can also pass along DOM elements, e.g., via an id MathJax.Hub.Queue(["Typeset",MathJax.Hub,"mathId"]);
. See the documentation on modifying math on a page for more details.