Microsoft made sure to curtail Adobe Flash's activities in Internet Explorer 10 Metro, limiting usage of the plug-in to websites the company has verified. While this benefits the security and performance of the browser, it's just annoying if your favourite Flash-powered website isn't included in IE10 Metro's whitelist. So here's how you get around that.
A post on xda-developers details the process you'll have to go through. First, you'll have to disable the automatic compatibility updates in IE10 Metro so your altered whitelist isn't randomly obliterated. This can be accomplished by loading the browser, pressing Alt to show the top toolbar, then selecting Tools -> Compatibility View Settings. Uncheck the option "Download updated compatibility list from Microsoft".
Now you can edit Microsoft's Flash whitelist. The XML file you need to edit is located in "%HOMEPATH%\AppData\Local\Microsoft\Internet Explorer\IECompatData\". "%HOMEPATH% is an environment variable that points to the current user's home directory, so you can just copy / paste the aforementioned path and Windows will automatically take you to the correct location.
Once the folder is opened, find the file "iecompatdata.xml" and be sure to open it in Notepad or XML editor of your choice. Then, find the section that begins with "<Flash>" and follow it to the end, marked by "</Flash>". Just before the closing tag, insert the name of the domain you wish to enable Flash on. For example, to add Lifehacker, you'd do the following:
[...] zumiez.com zynga.com lifehacker.com.au
The last step is to delete your browsing history in IE10 Metro. Once that's done, you should be able to hit up the site you entered and have Flash operate as it was intended.
[HACK] Flash on Any Website in IE10 Metro Browser [WIN 8 & WIN RT] [xda-developers, via AddictiveTips]