There are many ways to bring your sequences into MacVector.
When you start MacVector for the first time you will be presented with the Starting Point Dialog. This allows you to create a New MacVector file, to open a recently opened file or to Open an existing file
Usually this will just work. However, this is always reliant upon the operating system deciding which application to use to open a file. If you have multiple sequence analysis applications installed, or your file has no file extension you may find that the file opens in the wrong application. If you right click and choose OPEN WITH you can choose which application to use.
This is the traditional, always works, option. You do not have to worry about file extensions as well. MacVector will look inside the file and try to automagically use the most appropriate file format importer.
If you have copied any sequence data to the clipboard then you can directly open this using the New From Clipboard function. This also recognises Genbank formats if you have copied a file directly.
If you have a blank sequence and no existing curated sequences you can use Import Features from GFF/GFT and BED files to bring features in from one of many Genome Browsers that allow you to export date in a BED/GFT or GFF file. This function allows you to annotate an unannotated or partially annotated sequence with annotations (or features) contained within a BED/GFF/GTF/GFF3 file. Don't forget the Auto Annotation. tool scanning your sequences against your own annotated ones either.
MacVector has an Entrez browser that lets you search the online Entrez GenBank database using keywords and retrieve matching sequences either To Disk or as sequence windows (To Desktop) without needing to leave MacVector. You can access this via the Database | Online Keyword Search for Sequences (Entrez)? menu item.