"at a conference" is correct and can be used in written English. You can use it when you are talking about a specific conference that has taken place or will take place in the future. For example, "I will be presenting my project at a conference next week.".
Speak at a conference,"' she said.
My husband's at a conference.
Recently I was at a conference.
Most found seats at a conference table.
She recalls meeting Mr. Brown at a conference.
And at a conference a lecturer referred to Zorn's Lemma.
He will present the evidence at a conference on Thursday.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK