When you use type="text/plain", you can't add any TiddlyWiki formatting codes. If you change to using type="text/vnd.tiddlywiki" or leave the type field blank, then the tiddler content can use TiddlyWiki formatting codes, such as "doubled single quotes" around text to make it appear bold.
However, using TiddlyWiki 5 formatting will also affect the use of newlines within the content, and your text will automatically "flow and wrap", rather than keeping blank lines as-is. To retain the look and layout of "text/plain", you can enclose your text within some CSS, like this:
@@white-space:pre;font-family:monospace;border:1px solid #ccc;background:#eee;
... your content here ...
... including ''bold formatting'' as desired ...
@@
Note that, at a minimum, only the "white-space:pre" CSS is needed; the font-family, border, and background CSS attributes are optional.
enjoy,
-e