I downloaded this plugin without changing the default configuration, but I don't have any scarecrows in the entire map. I have no error messages when loading.

Hi,

As written in the description of the plugin : it does not spawn anything, it just modify spawned scarecrows spawn through the halloween.scarecrowpopulation server var that set the amount of scarecrows per km².

Spiikesan

Hi,

As written in the description of the plugin : it does not spawn anything, it just modify spawned scarecrows spawn through the halloween.scarecrowpopulation server var that set the amount of scarecrows per km².

Ok, thanks. 
Is there currently any plugin to add "zombies" or similar to the game?

Hi,

You just have to use the convar in the console with specified amount and then writecfg to keep it. like :

scarecrowpopulation 20
writecfg