Apparently GMS1 function list didn't even list the ds_map_set function, which is used for m[?k]=v accessor.
You can workaround this by adding a script called, say, scr_ds_map_set that'd do
ds_map_set(argument0, argument1, argument2)
and then adding
gml_func_add("ds_map_set(id, key, value)", scr_ds_map_set);
to the end of GMLive's create event.
I'll fix this in the next update.