I personally find it easier to use the vertex groups panel in the properties editor mesh tab. Select the vertices in edit mode. Select the vertex group with the same name as the bone. Set the weight and press assign. If you want to stay in 3D viewport you can use the hotkey Ctrl+g while in edit mode. So select vertices. Ctrl+g->weight to set the weight. Ctrl+g->set active group->select bone name. The Ctrl+g->assign or it may say assign to active group.
