Only if you are the owner of the secret RSA key d you are able to decrypt messages which are encrypted with the corresponding public key. If you just have access to the public RSA parameter N and e you can try to break the RSA key (compute the private key, works only for short keys) via the factorisation attack.
To decrypt RSA-encrypted messages, enter the numeric code of the encrypted message. When you click on Decrypt, the entered cipher text
c = c[1] # c[2] # .... # c[j]
is decrypted using the secret key (N,d) according to the following formula:
m[i] = c[i]^d mod N for i = 1, ..., j .
The numbers m = m[1] # m[2] # .... # m[j] are then transformed to the original text message M undoing the block splitting: