Judging by the documentation I was under the impression that this was a default property that all edges would have. Is this not the case? How would I add it?
I already tried putting a "double edge_weight" field in my EdgeProperties struct, didn't help.
There are no default edge properties - no default vertex properties either (with a few exceptions).
If you are using bundled properties, then you have to wrap the access to your edge weights with a property map and pass that map to various algorithms. For example:
typedef property_map<Graph, dobule EdgeProps::*>::type WeightMap;
WeightMap wm = get(&EdgeProp::weight, g);
Edge e = // get some edge descriptor from g
assert(g[e].weight == wm(e));
Passing wm to algorithms will allow them to use your custom edge weights.