Hello Filipe, if you mean axisymmetry = rotational symmetry, please choose "Axial symmetry" when creating a new model in ATENA Engineering 2D (see the ATENA Engineering User's Manual, 2.1 Open New File (File | New) for more info).

If you mean plane symmetry, please see ATENA Engineering 3D Tutorial, 3.7 Supports and Actions for an example defining the boundary condition for a symmetry plane (we generally recommend to go through this tutorial before starting to create one's first own model).