That depends on what platform you are on.
First of all, on both Mac and Windows, you will need to turn on the Multiple Instances preference (this is under the Documents tab in the MoneyWorks Preferences dialog box). This is off by default, so the default behaviour when opening a new file is to close the file that was currently open.
Mac
With Multiple Instances turned on, you can open another file on Mac by choosing that file from the Open Recent menu (you will have to have previously opened it). This will cause MoneyWorks to start a new instance of the application for you.
As of v7.1 you can create a shortcut for starting new instances by entering moneyworks:// in a Safari window and then dragging that URL to the Dock. Clicking that shortcut will now start a new instance (provided the Multiple Instances preference is on, and assuming the current instance has a document open already).
Windows
On Windows, you can open a new instance of MoneyWorks by choosing MoneyWorks from the Start menu. You can then open another document. Or, you can double-click a document in Windows Explorer, and Windows will start a new instance of MoneyWorks for you. On Windows, the Open Recent menu does not start a new instance. If you double-click a report or form document, MoneyWorks will open that in the currently running instance, rather than starting a new instance, regardless of the Multiple Instances preference