Ah, OK, I think I see.
CSV is a very simple format — literally, comma separated values, i.e. lines of plain text with commas in between the fields. So things like special formatting that you've chosen in Excel do not get preserved when exporting as CSV. To see what the contents of your CSV are, you can open it in a "text editor" — on Windows, the built-in one is Notepad. On Mac, there's TextEdit. If you're using RStudio, its Source editor serves the same function (and is more full-featured). You can share the contents of your CSV by opening it in a text editor and copy-pasting a few lines here.
So, first question:
- When you open the CSV file in RStudio (click on it in the Files pane or open it via the File menu), or in another text editor, are the leading zeroes present?
If so, then the solution is fairly simple — you add some more details to the import code to make sure that variable is imported as text, instead of as a number (as it currently is). To get help with exactly how to do this, you'll need to post the code you used to import the CSV (there's more than one way to do that in R!).