Currently, on a 64-bit Mac (such as >=10.8) with Java 7 you cannot use Java Applets in Chrome.
Java 7 is 64 bit only, and as such requires a 64 bit browser. Chrome on Mac is currently only 32 bit.
The Chrome/Chromium developers have a bug to show progress, although actual work seems to have stalled for some reason.: http://code.google.com/p/chromium/issues/detail?id=115774
If you wish to use an older version that will work (not recommended due to security issues) you can follow these steps, assuming you still have java 6 installed: http://support.apple.com/kb/HT5559 (summarised below)
sudo mkdir -p /Library/Internet\ Plug-Ins/disabled
sudo mv /Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin /Library/Internet\ Plug-Ins/disabled
sudo ln -sf /System/Library/Java/Support/Deploy.bundle/Contents/Resources/JavaPlugin2_NPAPI.plugin /Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin
sudo ln -sf /System/Library/Frameworks/JavaVM.framework/Commands/javaws /usr/bin/javaws
To undo these steps
sudo ln -sf /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/javaws /usr/bin/javaws
Followed by downloading the latest Java from Oracle.