You can calculate the best-fit lattice plane through three or more selected atoms, using the Calculate > Plane Through Points command. Details of the fit results are displayed in the window log.
If you are working with a crystal, a lattice plane will be shown passing through the selected atoms. If, on the other hand, you are dealing with an isolated group of atoms (i.e., in "molecule mode"), then a lattice plane cannot be shown because there is no unit cell. If you wish to show a plane you can either:
Convert your molecule to a (dummy) crystal, using the Transform > Molecule to Crystal command; or...
Add a dummy crystal to the document and make this the primary selection (in the Structures List shift-click to ensure that both molecule and crystal are visible, then click on the crystal thumbnail so this has the yellow highlighted border). You can now execute the Plane Through Points command and the resulting lattice plane will be displayed in the dummy crystal.
See Also