Whenever you see the term mod it means the remainder you were taught in school. For example, 6 divided by 7 equals 0 remainder 6. SO let us choose primes 137 and 89. Both have remainder 5 when divided by twelve. The modulo of 137 to 89 is 48.

Let us choose plaintext value 13. Take 15 to the 15 power (89+1)/6=15. (I switched the encryption and decryption algorithms, sorry.) and take the remainder of that divided by 48. You get 32. Take 32^3 mod 48 and you get 15.