1. Try to fill Community tab with your forum login/pass and restart Metatrader:
2. Internet Explorer 8 or higher should be installed on computer for the Market to be worked in Metatrader.
3. If it does not help so you can look at two posts about how the other people fixed the similar issue:
Market Tab Error, show a blank page and missing optionhttps://www.mql5.com/en/forum/170760#comment_4177231 andhttps://www.mql5.com/en/forum/71467/page2#comment_9026436