When you Use the HTML tool, you drag a box out on your page where ever you want the item to appear. If you have an exact size, you can enter the width and height in the Properties box. Also, there's the option to Edit HTML (body and/or head). You can always add/change things as needed. Within VSD, you'll only be able to see that place-holder box, but in preview or after publishing, you'll see your widget.
Back in VSD, you can drag that box around or use the arrow keys to nudge it into place.
Would this be for some analytics code? If so, you can click on Edit then scroll down to Add Analytics Code and paste it in there.
Let us know if you get it worked out (or not)