I have not examined the zip file. I will leave that for you as exercise.
But I'll try to give quick advice how to proceed with finding key, in typical cases.
The first things would be to find out what cryptographic algorithm has been used to encrypt the file. Maybe the name of algorithm is mentioned (file metadata, or you know the program used to encrypt)? Is there anything that reveal key size?
If there is not anything obvious (like identifier of the encryption mechanism), try to look for differences between encrypted and original form, such as is there difference in length? Is it padding?
If there is padding applied, the size of padding may suggest the algorithm. If the file is padded to multiple of 8 bytes, the most common algorithms would be:
* DES, 3DES
if to 16 bytes:
* It is likely AES
(Maybe there is reason to expect some other cipher, if text is Russian, it could be GOST.)
Or maybe there is reason to suspect something weaker i.e. maybe the encrypted file does not appear binary at all (then it is possibly exercise with a substitution cipher, in that case continue reading here possible ways to crack simple substitution ciphers). Anyway, try to determine or guess the cipher.
Modern well-tested block ciphers like 3DES and AES using properly generated key are designed to be invulnerable (known plaintext will make it easy to test for solution, but finding solution will still take very long time as the key space is amazingly large) to known plaintext attack. If this is exercise work, you may expect some intentional weakness (like most key bits are zero). In case there is no reason to the material was made intentional easy to find the key, you can stop now. It is likely no use trying.
In case you expect DES and you're ready to spend quite a while, it is doable. If it is a toy or exercise cipher, you should get lucky.
In case you expect that the encryption key is actually based on a (possibly) weak password, then the most effective way to crack the encryption is likely to brute-force the password instead of the key. To do this you need to first determine the key derivation algorithm used.