I'm not sure, if it has anything to do with SVN, but you can do the following.
Revert all the changes made at richmemo package. (if you're using tortoise SVN you can just right click on it and hit select "svn revert")
Then - uninstall the package from Lazarus and recompile the IDE.
After that - install it richmemo again, recompiling IDE.
Once done, create a new project and drop RichMemo on the form. The only unit that should be added to "uses" section is "RichMemo" and not "RichMemoPackage"