Many molecular biology softwares do not distinguish between single strand DNA (ssDNA) and double strand (dsDNA). SnapGene 5.2 and later represents ssDNA as a unique file type and allows the importing or creation of ssDNA sequences.
A ssDNA cannot be cut by restriction enzymes and as such the Show Enzymes button and some functionality accessed via menu Enzymes is disabled when viewing/editing a ssDNA sequence.
You can convert a ssDNA sequence to dsDNA sequence via menu Actions → Convert to Double Stranded. Once a ssDNA sequence is converted to dsDNA you will be able to see/use restriction sites. See the User Guide chapter on ssDNA for more information.