On the Technical Analysis (TA) block look for Custom Indicator - iCustom. In Custom fields type the indicator name and all the custom indicator variables separated with a comma. You also need to change Symbol, Timeframe, Mode and shift accordingly. (See picture)
MQL4 iCustom has the following parameters: (From
http://docs.mql4.com/indicators/iCustom)
double iCustom( stringsymbol, inttimeframe, stringname, ..., intmode, intshift)
You need to follow the same format. For example, for an indicator called myIndicator that uses two variables add this in the Custom fields:
"myIndicator",2,3 where 2 and 3 are the variables values. If you do not add all variables or do not use the right format in Custom fields, Molanis Strategy Builder won't generate the code and you may see some errors.