You can use the browse button to search for your program. The drop down list provides recently run programs. The command line arguments can also be based on the status of the current document, and are always enclosed in $(...), where … can be:
FULL_CURRENT_PATH
the fully qualified path to the current document.
CURRENT_DIRECTORY
The directory the current document resides in.
FILE_NAME
The filename of the document, without the directory.
NAME_PART
The filename without the extension.
EXT_PART
The extension of the current document.
NPP_DIRECTORY
The directory that contains the notepad++.exe executable that is currently running.
>CURRENT_WORD
The currently selected text in the document.
CURRENT_LINE
The current line number that is selected in the document (0 based index, the first line is 0).
CURRENT_COLUMN
The current column the cursor resides in (0 based index, the first position on the line is 0).
For example, "$(NPP_DIRECTORY)\notepad++.exe" -multiInst "$(FULL_CURRENT_PATH)"
would start a new instance of Notepad++ opening the current file. Remember to put quotes around paths if it can contain spaces. For the Notepad++ command line options, see Command Line.