In InfoPath 2010, things are moved around a little bit…

In 2007, to get to Form Options, you’d click on the Tool Menu. Now, in 2010, click on the File page (in the ribbon). Then, under Form Information, toward the bottom, click on Form Optiions (the label beside the button says ‘Advanced Form Options’).

The first thing you need to do is click on the Compatibility category. Click the dropdown for ‘Form Type’ and choose ‘Web Browser Form’. Of course, then, you need to enter the URL of your SharePoint server running InfoPath Forms Services.

Then, the next time you open that same Form Options Dialog, under Categories, you will see ‘Web Browser’.