This happens while in the WYSIWYG view. The solution is to add some spaces after the personalization code while in plain text. You can switch to plain text mode by clicking the Blue link at the top-right corner of the WYSIWYG editor "Use Plain Text Editor." You'll add the extra spaces after the code that looks like this:

<p><convio:session name="1" param="first_name"></convio:session>

After adding the spaces, you can switch back to the WYSIWYG editor by clicking the pencil/paper icon at the top-right corner. You should now be able to place your cursor after the personalization component.