Apple has not publicly released the installation files necessary to enable Power Nap on the MacBook Air (late 2010) however, they do have the files freely available online! Downloading one of the following packages will allow you to enable the power nap setting under Energy Saver in System Preferences. Simply downlod the file and follow the on-screen instructions.
Note: Install these at your own risk, there might be a reason why Apple has not publicly pushed out these updates to all MacBook Air users. This works on the MacBook Air Late 2010 only.
If you have any questions, feel free to leave them in the comments below!