Insert a file path into an expression using a file browser instead of typing.

From the Menu...

To insert a file path into an expression, choose Insert File Path ...:

Out[2]=

In a Program...

Use SystemDialogInput to bring up a file browser when an expression is evaluated:

Notes

Insert File Path ... and SystemDialogInput give absolute file paths. To construct a file path relative to a notebooks directory, use NotebookDirectory and FileNameJoin: