By converting your old PostScript Type 1 fonts, you will be able to use them again with most modern apps that stopped supporting PostScript fonts.
First you need to locate your old fonts. If you already have them in a folder you are all set. If they are installed in your system, you find them by going to the "Finder", holding down the option key and choosing "Library" from the "Go" menu.
Once the "Library" folder opened, you will find your fonts in the "Fonts" folder. Change the window to "list view" and sort the files by "Kind" to find the "Post Script" fonts. The exact type can vary and will either be "PostScript LaserWriter Font" or "PostScript Printer Font".
Chances are FontConverter will be the only app on your system that can read legacy fonts. In those cases, you can just double-click your font files and they will be opened and previewed in FontConverter.
Alternatively, you can open FontConverter and press the "Open PostScript Fonts" button at the top. Hold down the shift key to select more than one font in the open dialog.
If the preview looks as expected, you can press the "Export OpenType Fonts to Folder" button. You will then be prompted to select a folder in which the OpenType fonts will be created.
Now you are done! Make sure that the converted font looks and behaves as expected and keep using it in your designs.