dunno how to activate martha event
If you don't mean the sex at night while she's in the bed, then you probably mean her State Event?
If the former, the comment below yours already answered. If the latter, you need to do all her other scenes at least once with the exception of the bathroom handjob. You need to do that one twice after she's at 70 Guilty points so she'll mention she's gonna get dressed and then you enter her room and have sex with her. Afterwards, she'll be near the fireplace at midnight.