First, you must ensure that there is a file in the directory you're adding, since git only operates on files. Since you've already done this, the next thing to do is to trouble-shoot.
Using git add . will only add from the current directory downward; if used, it must be from the root of the repository. Sometimes, git add --all will be easier and more appropriate.
You need to look for a .gitignore file. Notice there is a "dot" at the beginning of the file; on most operating systems (and certainly Windows), these files are classified as "system" files and are hidden by default; you can, however, make them visible in an operating-system-dependent manner. The .gitignore should be at the root level of the repository you cloned, but sometimes there may be more than one in different directories. Using a command-line or GUI tool may help find any .gitignore files that may exist.
If you're looking at a .gitignore file, be sure that you understand any regular expressions which may be inadvertently matching the file you're trying to add. You could, as a simple test, try renaming the .gitignore and trying the add again, but be ready to unstage the files that will be added as a result. You could also try git check-ignore -v <dirname>, or read the help for check-ignore for more advanced options. Some useful help on this is offered by git help gitignore.
If all else fails, try adding an arbitrary test file, to ensure that something else is not wrong with the repository.