if your not using the built-in GetSaveAsFilename dialog to get the user input you should consider using that. It allows you to define the file type list and you can change the current directory prior to displaying it. See the MS Excel VB Help on this for the GetSaveAsFilename Method.
To get only a path from a full path and file name string, Use a loop with the 'Mid' function to check each charter in the full path working backwords until you get a "\" backslash. Then use 'Left' with the loop count to get only the path less the file name.
You can use the 'Dir()' function to check for a valid path or path and file. using the getsaveasfilename dialog will eliminate many problems of bad user input.