Your data file is stored on your system as a 'YourFileName.moneydance' file.

On a Mac system, the file will appear with the Moneydance icon displayed (see below).
On a Windows system, the file will appear as a regular 'folder' so you won't see the Moneydance icon.

Moneydance will store any new data files you create in the same, default location.

If you have not manually moved your data file to a different location, you can access the file by navigating within Moneydance to Help > Show Documents Folder.

Any files stored in the default location can be accessed within Moneydance by selecting File > Open.

Using a Mac - after opening your data file you can right-click (Ctrl + click) the Moneydance title bar, where the file name is displayed.
You can then select the folder directly below your file name ('Documents' in the screenshot below). This will open a Finder window with your file displayed and highlighted.

The default location for your data file(s) varies depending on your operating system (and version) and is listed below - 

MacOS BigSur and later

MacOS X Lion -
MacOS X Catalina
MacOS X Snow Leopard (and earlier)/Users/<username>/Library/Application Support/Moneydance/Documents
Recent versions of WindowsC:\Users\<username>\.moneydance\Documents
Older versions of Windows
C:\Documents and Settings\<username>\.moneydance\Documents