Frost is an application for Freenet that provides usenet-like message boards and file uploading/downloading/sharing functionalities. It should get installed with Freenet 0.7 automatically if you used the standard Freenet installers.
To run Frost on Linux from the command line, change to your Freenet install directory, then change to the frost subdirectory. Then make the frost.sh file executable, and run it:
cd /home/username/ cd frost/ chmod +x ./frost.sh ./frost.sh
The first time you start Frost, you will get a number of dialogs. The first ask you if you would like to import old Frost data. If this is the first time you use Freenet at all, you can safely answer Clean startup here.
Since Freenet is able to run on both Freenet 0.5 and the current Freenet 0.7, you will need to specify which version you want to run with. If you have followed the previous install instructions, you will probably want Freenet 0.7 (Darknet).
Next step is to choose which identidy you want to be known by when using Frost. As it says, it does not have to be unique, but it certainly helps.
You will then get the main Frost window, where one can participate in the message boards and up/download files. The windows will probably take a while to populate on the first startup.
Sharing files can be done by clicking the Shared files -tab and then clicking on the Folder-icon top left in the appearing tab.
Additional information about Frost can be found in the mailing-list or on the official website: Frost Homepage