I am not entirely certain I know what your confusion is, but I'll try my best. The basic gist is when rolling your to hit rolls, any dice that end up as 6s trigger the warlord trait's effect.
However, because this edition has reintroduced modifiers to dice rolls, that's where the "+" part comes in. Say for example on battle round 3 or later she would gain a +1 to all hit rolls in the fight phase thanks to power from pain, so any dice that roll 5s would be modified into a 6 and thus they would also activate the warlord trait (and any 6s would turn into 7s, hence the + part).
Now this also means if you were to give the Blood Dancer warlord trait to a standard Succubus, while wielding her glaive she is given a -1 to hit penalty and thus any dice rolls of 6 become 5s and cannot trigger the warlord trait unless that penalty is offset (such as battle round 3 power from pain).