svnadmin dump is for working on the repository(server side), not the working copy (client side)
The errormessage about format is misleading as it searches for the format file in repository and finds a file with same name in your workingcopy.
If you are interested to get a full backup of your repository you can use svnrdump for this purpose. It is used the same way than svnadmin dump, however it is possible to get the dump from an remote repository:
use
svnrdump dump -r0:HEAD [URL_TO_YOUR_REPO] > Your_dumpfile.dmp
THIS IS A NEW SVN 1.7 FEATURE