I couldn’t find this information on Plurk‘s website, so I’ve put it here for future reference.

Bold:
Use double asterisks before and after a word: **word**

Italics:
Use an asterisk before and after a word: *word*

Underline:
Use double underscores before and after a word: __word__

URLs:
Type the URL and put the title you want in brackets: url (title)

Videos & Images:
Paste the direct URL into the text box

Link User Names:
Use the @ symbol before the user name to link to their profile: @Username