MacVector automatically scans every DNA sequence window for open reading frames and displays the results in the Map tab.
The setting for this are controlled by the MacVector | Preferences -> DNA Map pane, along with the automatic Show restriction sites settings.
The Minimum Number of Codons setting is fairly obvious. 5’ ends are starts and 3’ ends are stops are used for linear sequences and will display ORFs coming into the sequence that do not have a start codon, or ORFs that read out of the sequence without a stop codon. Codons after stops are starts is useful if you just want to know the locations of the longest possible ORFs in a sequence, irrespective of the existence of a start codon. Finally, the Suppress annotated CDS ORFs check box prevents the display of ORFs that are already annotated. However, if a potentially longer ORF is present, e.g. there is another ATG codon upstream, that ORF will be displayed. You can read more details here.
The appearance of the ORFs can be controlled using the Options | Default Symbols menu item.