I am working on a system which includes grid, wind, solar PV, and biogas produced from anaerobic digestion.  I am finding a problem to input the biomass resource in this case because it does not involve gasification as described in 'help' of Homer.  Any suggestions?

If you don't have to worry about scarcity in the supply of biogas, then I recommend you model the biogas as just another fuel.  Click the New button beside the fuel drop-down box, enter its properties, and call it "Digester Gas" or something.  Then you can specify a price just like for any other fuel.

If the biogas is available only during part of the year, then I recommend you use the Biomass Resource Inputs window.  It's no problem that the process doesn't involve thermal gasification like we had in mind when we created that window.  The window lets you specify the availability of a feedstock, and the ratio of gas produced to feedstock consumed.  That could apply to anaerobic digestion the same as it does to thermal gasification.