You can only enter keys if your receiver has been loaded with an emulator (emu) program. If you have been able to watch encrypted channels at any time without a key card inserted, then you already have an emu. In any case, if you haven't downloaded the latest emu version for a month or two, then you should because some keys don't work without it.
I translated the following from a post on a Turkish forum. Thank you to ozmena.com My Turkish isn't great, and I don't have a Topfield myself, so if you find the following inaccurate please post corrections.
===== Topfield Key Entry =====
Press Menu on the remote controller.
Choose "System Settings" (Or something equivalent) and press 121 to bring up a new menu item EDIT CODE. Select EDIT CODE and press OK.
Remote controller functions:
Red button: (Change CAS Type)
Green button: (Add): Enter new key
Yellow button: (Delete): Delete a key
Blue button: (Change code): Edit a key
Press red button on the remote controller to get list of encryption system types and select which one you want:
==== New Key Entry ====
Press green button (ADD). This opens a screen where we can use numeric characters 0-9, alpha characters A-F, DEL and OK.
The format of the key has three fields:
Provider (_ _ _ _ _ _) 6 digits,
Index (_ _) 2 digits,
Code Data ( _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ) 8 pairs of characters
Use either the remote control or pick characters from the screen to enter the key.
==== Example ====
To enter P*lsat keys for the first time.
Assuming that we have obtained the following information from a key file for two keys:
P*lSat Cyfr*wy 13°EAST HOTBIRD
DEC: 113 001
1- Press green button (ADD), to open new window.
2- Enter 007101 in the Provider field.
3- Enter 00 in the Index field, or 01 for the other key.
4- In the Code Data field enter the Hex alpha numeric codes.
5- Select OK from the menu to save the key.
6- Repeat for the other key.
==== To Change a Key ====
Using the P*lsat example above:
Press the blue button (Change Code). Scroll through the list of keys until you find the keys with Provider 007101. Select the key that has 00 in the Index field. Then use the same procedure as above, to over-write the data in the Code Data field.
Make sure you don't change the Provider or Index fields.