How do I access and enable more icons to be in the system tray?

From Ubuntu 13.04 onwards you have to whitelist the applications if you want to allow access to the system tray.

Firstly install dconf-tools from the software center ( or by clicking here) Then Press Alt+F2 and enter dconf-editor and run it.

Now navigate to Desktop -> Unity -> Panel.

Now change the value of systray-whitelist to

"all"

Note that setting this to "all" will likely lead to other bugs, as the old notification area is unmaintained, instead consider adding applications you need individually instead of just enabling everything. You can reset it to the defaults by checking out this question:

• How do I set the panel whitelist back to the default?

Do note that the system tray is entirely removed in 13.04 and newer and that older applications that still have not been ported need to be updated:

• Why aren't certain indicators showing in Unity?

In Terminal, copy and paste this command

gsettings set com.canonical.Unity.Panel systray-whitelist "['all']"

The tip comes from this website;

http://www.webupd8.org/2011/04/how-to-re-enable-notification-area.html

You can upgrade Dropbox, as in the latest builds it has an indicator on Ubuntu. For other applications, if the developers do not add indicator support for them, you can simply white-list them as Alaukik says, and also notify the developer that they need to fix the application.