Join our CAD community forums where over 25,000 users interact to solve day to day problems and share ideas. We encourage you to visit, invite you to participate and look forward to your input and opinions. Acrobat 3D, AutoCAD, Catia, Inventor, IronCAD, Creo, Pro/ENGINEER, Solid Edge, SolidWorks, and others.
Create the symbol, if you haven't already. Open an drawing and choose Insert, Symbol, Custom. Choose a symbol (any). In the dialog box that opens, choose New. I won't go into the details of how to define the symbol.
Then, add a copy of that symbol to your drawing and close out of the symbol dialog if you had it open. Go to Insert, Symbol, From Palette. The symbol instance palette opens. You'll see a button marked Add to Palette. Choose it, choose your symbol, and locate it in the palette by moving your mouse into the palette window.
The palette is just a drawing file full of symbols, so you could also edit it directly, if you chose. In that same Symbol Instance Palette window, choose Open to find out where the palette file is.
Here are a couple more hints that might help, too. You can set the location of your symbol instance palette in your config. Choose Tools, Options and set symbol_instance_palette_file to your custom palette, which is simply a drawing file containing your custom symbols.
You also have a symbol directory for the symbols that you get from choosing Insert, Symbol, Custom. You can save your own symbols to that directory. You can set the config.pro option pro_symbol_dir to the location of your symbol directory.