Dear Luis,
Adobe changed and now has separate instructions for several different browsers.
If you follow those steps for Adobe 10, you reach a different screen for Adobe XI.
Try clicking on the link on that screen "How to set up your browser to use Adobe products to view PDF documents":
http://helpx.adobe.com/acrobat/using/display-pdf-browser-acrobat-xi.html"When you click a PDF link on a web page, it can open either within the web browser or directly in Acrobat or Reader. Each browser has its own settings to control how PDFs open from a web page. Acrobat XI and Reader XI do not include a preference setting to open web-based PDFs. To change the display behavior, follow the instructions below for your browser, or see the browser documentation on how to control plug-ins or add-ons."
Internet Explorer 8 and 9
Google Chrome
Mozilla Firefox
Apple Safari
Cheers,
Carl Fogel