The Web Importer has recently been redesigned, it requires access to cookies on your machine with your login details. Unfortunately this has caused a problem with some user's internet browsers. If you are having this problem, you will need to allow cookies on your internet browser for www.mendeley.com. To do so, please follow the instructions for your browser below:
Click the 'Tool's button in the top left hand side of Internet Explorer (looks like a little cog in Internet Explorer 9) and click 'Internet Options'. Then, click the 'Privacy' tab, then 'Sites'. Under 'Address of Website', type in www.mendeley.com and 'Allow'. Click 'OK' on both screens. You may need to restart Internet Explorer afterwards.
For more information you can have a look at Internet Explorer 10's page on managing cookies (or see the alternative pages for Internet Explorer 9 or Internet Explorer 7 or 8)
Click the orange 'Firefox' button at the top left and click 'Options'. Then, under the 'Privacy' tab, either set the 'History' drop down option to 'Remember History' (which allows cookies for all sites), or tick 'Accept Third Party Cookies'. Alternatively, you can add www.mendeley.com to the 'Exceptions' box. Click 'OK' to save.
For more information you can have a look at Mozilla Firefox's page on enabling and disabling cookies.
Click 'Customise' (three black lines on the top right of the screen), and then 'Settings'. Click 'Show Advanced Settings' at the bottom of the page, then click 'Content Settings...' under 'Privacy'. Then, under 'Cookies' make sure 'Block third-party cookies and cite data' is unticked. Alternatively, you can click 'Manage exceptions...' under 'Cookies' and allow www.mendeley.com, then click 'Done'.
For more information you can have a look at Google Chrome's page on cookies and site data.