Cannot Commit to the SVN – CHECKOUT can only be performed on a version resource [at this time]

You can do an interactive rebase and choose edit for the commit whose date you would like to alter. When the rebase process stops for amending the commit you type in for instance:

git commit --amend --date="Wed Feb 16 14:00 2011 +0100" --no-edit

P.S. --date=now will use the current time.

Afterward, you continue your interactive rebase.

To change the commit date instead of the author date:

GIT_COMMITTER_DATE="Wed Feb 16 14:00 2011 +0100" git commit --amend --no-edit

The lines above set an environment variable GIT_COMMITTER_DATE which is used in amending commit.

Everything is tested in Git Bash.

I'm not sure what you're trying to do: If you added the file via

svn add myfile

you only told svn to put this file into your repository when you do your next commit. There's no change to the repository before you type an

svn commit

If you delete the file before the commit, svn has it in its records (because you added it) but cannot send it to the repository because the file no longer exist.

So either you want to save the file in the repository and then delete it from your working copy: In this case try to get your file back (from the trash?), do the commit and delete the file afterwards via

svn delete myfile
svn commit

If you want to undo the add and just throw the file away, you can to an

svn revert myfile

which tells svn (in this case) to undo the add-Operation.

EDIT

Sorry, I wasn't aware that you're using the "Versions" GUI client for Max OSX. So either try a revert on the containing directory using the GUI or jump into the cold water and fire up your hidden Mac command shell :-) (it's called "Terminal" in the german OSX, no idea how to bring it up in the english version...)