Ok, here's what you do:

Click on the "Account" link in the upper left-hand corner
Click on "Review Program"
Click on "Product Search"
Select "For Free Assignment"
Select either "Eden sponsored" or "Manufacturer Sponsored"
Then in the drop down box where it says "With or without reviews", select "without reviews"

You can also fiddle around with price ranges and toy types. But, it's not very intuitive so you have to be very broad with your search terms.

All lingerie and costume items are off-limits for free assignments, but can be purchased as buy-outs. If your request is denied it's probably because it's a first come, first served situation and the toy could be in the hands of or on the way to other reviewers. Don't sweat it. There's plenty of stuff to choose from. Also, if a product does have reviews but you feel you can write a better review, you can submit the request with your reasoning and see if it gets approved.