Your manager should know the following:
- That if there is a problem you will always come to him/her first not try and work around him
- That he can count on you to do what needs to be done. You will go out of your way for her. If you've ever said "this isn't my job" then you will have only an adversarial relationship.
- That you understand what his/her boss wants, and therefore can exercise good judgement in using whatever leeway you are asking for.
Of all of these, that last point is the most important. You should intimately understand why they consider a conference talk to be something negative. Your manager should know you understand that and can therefore trust you. The fact you don't understand why they are against this conference talk is the root of the failure.
Note that the relationship should be reciprocal. I'm not saying be a slave to your boss, but if you want anything other than the traditional boss/subordinate relationship you need to have trust you have each others backs.