Did you install opencpn from source or a deb package?
When you installed libtinyxml did the installation
complete without an error message?
Precisely what command did you give when trying to start opencpn from within a console?
If you installed libtinyxml from a deb package you should be able to locate libtinyxml.so.2.6.2 in /usr/lib. Simply navigate to /usr/lib and scroll down. If tinyxml is installed you will most likely find it here. If you don't, then tinyxml is probably not installed. Also look in /usr/local/lib.
When you say you received the 'illegal command" next time you "ran" opencpn in the consolse, are you talking about opening a previously installed, working opencpn? Or, are you talking about when you were trying to install opencpn from source?