To install an AMXX plugin (assuming lasersight is AMXX not metamod plugin) do what you did before... save the source (.sma file) into addons/amxmodx/scripting/ and compile.
Compiled .amxx plugins will be in the scripting/compiled/ folder, and you need to move or copy them to addons/amxmodx/plugins/. Then make sure the plugin is listed in addons/amxmodx/configs/plugins.ini on it's own line with the filename of the .amxx compiled plugin.
You can check if your plugin loaded successfully with either:
Server console...
Client (w/ADMIN_RCON) console?...:
Code:
amx_plugins
Or if that fails... one of these two:
amx_showrcon amxx plugins
amx_showrcon "amxx plugins"