If you see rule #3 of this community, it mentions that support question should be posted at /c/[email protected].
Search depends on what you would like to find: from the web ui, you have to select the type (comments, users, posts), then choose the scope (all, local, subscribed), then choose the sort type (top all time, controversial, new, old, etc).
The rest of the forms (like community, creator) are self-explanatory - community drop-down won’t work if you’re searching for a community, likewise creator drop-down won’t work if you’re searching for a user.
To ping and mention a user, @<user>@<instance>.<domain> (e.g.- @Mantikora ) should work. To just mention them, /u/<user>@<instance>.<domain> (e.g.- /u/[email protected]) should do. To point a specific community, /c/<insert-community-name> (e.g.- /c/[email protected]) should work.