This is probably not good enough for a how-to on plone.org, but worthwile recording anyway. I had a bit of a hard time installing plone.app.form on Plone 2.5. It's required by slideshowfolder 4.0, which has some nice extra features compared to 1.2.2.
Like the instructions in slideshowfolder 4.0 told me, i got plone.app.form for Plone 2.5 from http://svn.plone.org/svn/plone/plone.app.form/branches/plone-2.5. The instructions don't tell you to build an egg from this, like so: python2.4 setup.py bdist_egg The egg will go in the dist/ directory.
The best way to install this in your Zope instance (not in your system-wide python2.4) is to create a workingenv for your instance, as indicated in http://plone.org/documentation/how-to/zope-python-packages-using-easy_install. Then you can activate your workingenv, and use easy_install, except you don't easy_install plone.app.form, but you point easy_install to your created egg: easy_install-2.4 /path/to/dist/plone.app.form*.egg. It'll probably complain about a site.py already being there, just remove it and run easy_install again.
Finally, do modifications to $INSTANCE_HOME/etc/site.zcml, $INSTANCE_HOME/etc/site-packages/configure.zcml and $INSTANCE_HOME/etc/site-packages/overrides.zcml as indicated in the plone.app.form checkout's docs/INSTALL.txt.