<a onClick="javascript:x("<b>","Please enter text to make bold:","</b>")">Bold</a>
should be <a onClick="javascript:x('<b>','Please enter text to make bold:','</b>')">Bold</a>
Or in other words, try changing all the quotation marks within the main ones to apostrophes in the link.
Code:
<a onClick="javascript:x('<b>','Please enter text to make bold:','</b>')">Bold</a>
What it was doing was seeing the second " (after x) and closing the 'onclick' variable. Using apostrophes instead won't cut off the opening quotation mark until you want it to.