Installation was painless in wordpress 3.0 and the button appeared correctly in picasa 3.6.6 on OS X Snow Leopard just fine.
First use presented me with the login page but when I entered my credentials nothing happened. Using the "go back to ... " link in the top left took me to the homepage and I was logged in though.
Uploading worked fine, with one minor problem. If there is an apostrophe ' in the description, the description text gets cut at that point so if my description text read "apostrophes in descriptions aren't parsed" the description in the photo would be "t parsed"
I haven't had a chance to test this with other characters, like " ` etc.