When you select the option to convert to symbol, you can see two check boxes with one of th options being Save to common library.
You can follow the following steps:
1. Create a rectangle.(or the design that your symbol willl have.)
2. Click on Modify>Symbol>Convert to Symbol or Right-click convert to symbol or press F8 to convert the rectangle to a symbol.
3. In the Convert to symbol dialog, check the "Save to Common Library" option.
4. You'll get a dialog to save the symbol. It is saved in Custom Symbols by default.
The Common Library panel will be refreshed and you can see this in the "Custom Symbols" section inside the common library.
I'm glad that it worked for u, Liz