First, make sure you have cloned the SVN repository with SmartGit/Hg, not with git-svn (usually SmartGit/Hg will warn you about that when opening such a repository). Then:
- use Branch|Checkout to move to the revision from which you want to fork your branch,
- use Branch|Add Branch to create your new branch and select to switch to this branch,
- use Remote|Push. SmartGit/Hg will ask you how to push. Select Push as New here.
Now, your new branch will be present in the SVN repository and should show up in SmartGit/Hg's Log as a remote branch as well.