When radio buttons don't work it's usually due to the browser you are using. Please try some troubleshooting. Which browser and version of it are you using? Have your tried clearing your browser cache and cookies? http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=32050
Is your browser version up to date? You can upgrade any browser version here.
Do you have third party cookies enabled? Do you have another browser you can try?
You can use this link to see if your Flash is up to date. http://www.adobe.com/software/flash/about/ If it’s not up to date then this is where to download and install the latest version of Flash. http://www.adobe.com/products/flashplayer/
How are you connecting to the internet and to WordPress.com? The way you connect to the internet (mobile, satellite, DSL, dial-up) and to your blog, and how many proxy server jumps etc. it takes to connect can cause problems. There maybe be problems with your ISP and/or with the proxy servers.
Are you using http:// or https:// see http://en.support.wordpress.com/https/
Have you had any recent operating system and/or security updates? Sometimes operating system and/or security system updates can cause problems so check for known issues. When you make adjustments to your browser settings — changing privacy settings, deleting cookies or increasing security — you will need to close your window and restart the browser before those new settings become effective.