I fiddled around with Simdock and it indeed seems to be broken, as it doesn't save the newly added icons.
A workaround is configuring the config file yourself. It is located in your home directory in a hidden folder called .SimDock
You can open it like this (on GNOME):
There you have to add new launchers like this:
You see, a new launcher begins with the <SimDock> tag and ends with </SimDock>
<description>Firefox web browser</description>
All you need to add is the path to the application (in most cases in /usr/bin) but you do not have to give the exact path.
As you can see gnome-terminal is enough if it's in /usr/bin
It's vital to add an icon path, though. Nearly any image filetype will do (except for .svg, which is actually pretty unfortunate).
Many application icons are stored in /usr/share/pixmaps or under /usr/share/icons
Hope this helps.