ok well firstly there are 4 parties to a crime
they are:
1.
Principle in the first degree-person who actually commit the main offence
2.
Principle in the second degree-person who was present, assisting in committing the offence
3.
Accessory before the fact-person who helped plan the offence but were not there when it occurred
4.
Accessory after the fact-person who knowing about the crimes, helped the offender afterwards
and the definitions of actus reus and mens rea are as follows:
Actus Reus-the guilty act- the criminal action.
Mens Rea-the guilty mind. Mental element of a crime. Proved by intention, recklessness
so in response to your question: How are Mens Rea and Actus Reus present in accessory before the fact and accessory after the fact?
the person who helped plan the offence has the mens rea in accessory before the fact as they take a part in helping plan out the crime
the person who knew about the crimes and helped the offender afterwards has both actus reus and mens rea as they are actually aware of it happening and assisting
hope that helped