I'm not a heavy KDE user, but one way is:
- Open the Menu;
- Switch to Applications tab;
- Right-click the Applications tab, and select "Edit Applications";
- Create a new item called Seamonkey in the Internet subsection, and point the command to run to the binary you already know;
- Now from the Menu > Applications > Internet section, right-click the Seamonkey item you created earlier and you can choose to add it to the panel.
Linux Mint 14 Nadia / 64-bit / Cinnamon