Further clarification: In the FAQ it says (about version 2.0, though) that you cannot import private keys to a seed wallet, the same thing was said in the previous topic where i asked for help. It was said instead that i should do as it is in the FAQ as well, creating a new wallet by restoring a wallet from seed, with the private key typed in as the seed. However, it simply disappears after pressing next and no new addresses have appeared in my electrum client, as if i did not do the whole process at all.
Seed is not the same as private key. Restore is done using seed not a private key.
Please answer my question above about what you are referring to as a private key. Is it really a private key or something else?