If you can log into the forums but not to NMM, you may have a non standard character in your password. Passwords for NMM are more strict than the rest of the site.
Please update to the latest version FIRST. this is a beta program and we do not provide ANY support for previous versions.
Clear your browser cache - Do this before posting - This fixes a lot of problems.
In Firefox press ctrl+F5. For other browsers check the 'help' to see how. This fixes a lot of problems - It is caused by your browser loading a previously saved copy of the page instead of getting a new copy
Change your password
MUST be only English characters and numbers (upper or lower case are fine)
No non English characters ( such as ç or ű )
No special characters (& # etc)
No dash, underscore or spaces (- _ )
At least 6 characters
Edit - some new info. Several people say they have been able to get it to allow them when they typed their password into notepad, (Or somewhere else) then copied it and pasted it into the NMM password field. Sounds strange to me, but maybe worth trying.
Edit: The problem may something like a firewall, antivirus, popup blocker, proxy or some other filtering software that doesn't like NMM. I have even see one instance of an ISP blocking a site.
Edit: 11-20-2012: A newer version of NMM changed the password section - trying to fix some of these password problems - But you MUST have the latest version - and if you cannot log in, you MUST download and install the newest version from the Nexus First.
Most firewalls & Antivirus programs have an exceptions list buried in their menu structure. Popup blockers usually have a 'Allow this' by a key or keyboard combination. Not having used a proxy in years, I can't help there.
You may try a different browser, preferably a portable version of one. Most popular browsers have a portable version that does not have to be installed on your computer to work, and can be run from a USB stick. You do not need to uninstall your regular browser to use this for testing. It can be used as is without loading ANY add ons, ad blockers or anything else. Download it from a site, unzip it onto a USB. Close your regular browser and start the portable browser. Here is a legitimate link for Portable FireFox. Most other browsers have one also.
Edit: December 3, 2013: For those that look at this topic and think there are an awful lot of people having this problem - This topic is well over a year old now. We have nearly 6.5 million members and 99% of them are not having a problem. This topic is for those that are - so please read it to find how others have fixed the problem - and please post back when you find your problem to help others.