Translation:You should log in to the instant messenger.
Why "Vi devus ensaluti la tujmesaĝilon" and not "Vi devus ensaluti AL la tujmesaĝilon" ou "Vi devus ensaluti je la tujmesaĝilon" ?
I don't think there's a good answer for this besides "because '(en)saluti' is transitive".
Why are you afraid OF something and not afraid FROM something? Why do you marry someone and not marry WITH someone? Why do you answer a question and not answer TO a question?
It is possible to say “al la tujmesaĝilo” and even “je la tujmesaĝilo” but "al" and “je” do not take an accusative of direction. "al" does no need it because it expresses direction by itself; “je” does not take it because it does not express a location.
The fact that “(en)saluti” is transitive does not mean that you cannot replace the accusative with a preposition.