How can I upload a Hot Potatoes Activity to my blog or website?

Unfortunately, there's no simple way (yet) to do so.

Here's the 'not so simple' way. 
Upload your html file to public folder of your Dropbox.com account.

To post in on your blog, RIGHT click/Copy public URL.
Then copy that URL into this code

<iframe src="URL Here" frameborder="0" height="500" width="600" scrolling="auto"></iframe>

and post that (using the 'edit html tab, not compose tab)  on a blog post