When i try to add new icons,kdm themes, plasma widgets or any other through the system settings add-on installer, it is getting listed as downloaded and installed in the addon installer but the particular downloaded theme is not visible in the text space. This issue is not in all the cases.

For example, when I download 10 loginscreen themes, all 10 are getting installed but only 2 are showing in the textbox list to choose and apply.

I am using Arch Linux and I don't use sudo. Is it because of this ?

Please advice

Plasma Applets and Icon Themes should install without problems, and do not require root access. Login Screen Themes on the other hand do require root access.

Can you please check the permissions on your home directory, in particularly under ~/.kde4 or ~/.kde.

The below is the permissions in my home folder

$ ls -ld .kde*
drwx------ 6 althor users 4096 28.06.2012 16:06 .kde4/
-rw------- 1 althor users 343 29.06.2012 11:27 .kderc

But I can manually download and install the theme files from the system settings addon installer

Those permissions seem fine. Can you reproduce this under a new user?

Okay, I will create a new user and try there

I am having the same problem on Arch. I installed several login themes through "Get New Themes", and while they show up as installed, I cannot select them to apply from the text list. First noticed it in KDE 4.9.0 but it could have been around longer.

$ ls -ld .kde*
drwxr-xr-x 3 holly users 4096 Jul 25 14:35 .kde
drwx------ 6 holly users 4096 Jul 25 14:38 .kde4
drwx------ 7 holly users 4096 Jul 25 14:27 .kde4-broken
-rw------- 1 holly users 350 Jan 21 2010 .kderc
drwxr-xr-x 5 holly users 4096 Oct 21 2011 .kdevduchain

Please check under the ~/.kde* folders (both your user and root) for the KDM theme.

Once you have found it, search for a kdm/themes folder structure under /usr. On my system, this is at /usr/share/kde4/apps/kdm/themes/. Then move the KDM theme you found under ~/.kde* there.

I don't think the themes are being downloaded for me.

~ $ find .kde* | grep kdm
.kde4/share/apps/knewstuff3/kdm.knsregistry

The same command on /root is empty, as is searching for the name of a theme I downloaded. Searching for theme only gives me my desktop themes. Only the themes that I installed via pacman (from packages kdebase-workspace and archlinux-themes-kdm) show up in /usr/share/apps/kdm/themes/

Not sure if this is related, but I thought if I tried running systemsettings as root, and installing the kdm themes from there it might work. However, sudo systemsettings gave this error:
$ sudo systemsettings
QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
systemsettings(5987): KUniqueApplication: Cannot find the D-Bus session server: "/usr/bin/dbus-launch terminated abnormally with the following error: No protocol specified
Autolaunch error: X11 initialization failed.
"

systemsettings(5986): KUniqueApplication: Pipe closed unexpectedly.

System Settings is not designed to be run as root.
You may wish to try manual installation, by finding and then downloading it from kde-look.org, then unpacking it in the location I indicated previously. (/usr/...)

Yup, downloading and extracting themes straight to /usr/... works. Should I submit a bug about this? To KDE or Arch?

Please submit a bug against the KDM control module on bugs.kde.org.

Looks like there already is a bug: https://bugs.kde.org/show_bug.cgi?id=255453

Also, if I chmod o+w the kdm themes directory, I can install themes through "Get New Themes".

Thanks for your help.

RIght, that is expected. What is needed is a implementation of a KAuth agent to allow permitted users to install them without needing this.