It is started automatically after addon is installed. But installation takes some time - just wait until you see message in kodi that lamp addon is installed.
Some instructions: OpenELEC Mediacenter - OpenELEC Forum - LAMP addon - Linux, Apache, MySQL PHP bundle (1/30)
If for some reason daemons doesn't start you can see some info with
systemctl status httpd
systemctl status mysqld
journalctl -a
Status httpd i get this.
httpd.service - Apache Httpd Server
Loaded: loaded (/storage/.kodi/addons/http://service.web.lamp/system.d/httpd.service; enabled; vendor preset: disabled)
Active: activating (auto-restart) (Result: exit-code) since Sat 2017-03-04 21:02:06 CET; 1s ago
Process: 2918 ExecStart=/bin/sh -c exec sh /storage/.kodi/addons/http://service.web.lamp/bin/httpd.start (code=exited, status=127)
Mar 04 21:02:06 LibreELEC systemd[1]: httpd.service: Control process exited, code=exited status=127
Mar 04 21:02:06 LibreELEC systemd[1]: Failed to start Apache Httpd Server.
Mar 04 21:02:06 LibreELEC systemd[1]: httpd.service: Unit entered failed state.
Mar 04 21:02:06 LibreELEC systemd[1]: httpd.service: Failed with result 'exit-code'.
Status mysql i get this:
Unit mysql.service could not be found
journalctl -a i get this:
Mar 04 21:04:09 LibreELEC systemd[1]: httpd.service: Service hold-off time over, scheduling restart.
Mar 04 21:04:09 LibreELEC systemd[1]: Stopped Apache Httpd Server.
Mar 04 21:04:09 LibreELEC systemd[1]: Starting Apache Httpd Server...
Mar 04 21:04:09 LibreELEC sh[4009]: /storage/.kodi/addons/http://service.web.lamp/bin/httpd.start: exec: line 114: /storage/.kodi/addons/http://service.web.lamp/bin
/httpd: not found
Mar 04 21:04:09 LibreELEC systemd[1]: httpd.service: Control process exited, code=exited status=127
Mar 04 21:04:09 LibreELEC systemd[1]: Failed to start Apache Httpd Server.
Mar 04 21:04:09 LibreELEC systemd[1]: httpd.service: Unit entered failed state.
Mar 04 21:04:09 LibreELEC systemd[1]: httpd.service: Failed with result 'exit-code'.
Mar 04 21:04:13 LibreELEC systemd[1]: mysqld.service: Service hold-off time over, scheduling restart.
Mar 04 21:04:13 LibreELEC systemd[1]: Stopped MySQL server.
Mar 04 21:04:13 LibreELEC systemd[1]: Starting MySQL server...
Mar 04 21:04:13 LibreELEC sh[4026]: Running first time table setup
Mar 04 21:04:13 LibreELEC sh[4026]: /storage/.kodi/addons/http://service.web.lamp/bin/mysqld.start: line 63: /storage/.kodi/addons/http://service.web.lamp/bin/mysql
d: not found
Mar 04 21:04:13 LibreELEC sh[4026]: start mysqld
Mar 04 21:04:13 LibreELEC sh[4026]: Setting up root password...
Mar 04 21:04:13 LibreELEC sh[4026]: /storage/.kodi/addons/http://service.web.lamp/bin/mysqld.start: exec: line 75: /storage/.kodi/addons/http://service.web.lamp/bin
/mysqld: not found