[Radek]

1. Copy the .esp file into the data files folder
2. Start the game and click "Data Files" in the opening menu.
3. You will get a list of all .esp's in the data file folder. Select the one with the race and double click it. You will see a check mark at it.
4. Leave the Data Files list and start a new game. The race should be available during creation of the player character.

Note: if you want to remove the race from the game, proceed the same way. After double clicking, the check mark will vanish, etc.