Hello.
The “+” is a Add Action and if you right click on the Graph you can find in under the category Math → Float or you can Right Click in the Graph and hit the “+” key on your keyboard and it will sort based on the search.
For an even narrower search you can click on a output node drag out the connection line and when you release the mouse click the Blueprint Action menu pops up, if you enable the Context Sensitive it will show you only the Actions connected to the output node you clicked.
That last method is a very good way to find something. Say you want to set a rotation of an object but you don’t know what the Action is called in UE4. Then I usually just right click in the Graph and starts typing what I want to perform, eg “Setrotation” and the system will narrow the search for my need.