Include:
#include <hamsandwich>
In plugin_init():
RegisterHam(Ham_Think, "grenade", "fw_ThinkGrenade")
Somewhere else:
public fw_ThinkGrenade(entity) {
// [...]
}
Taken from:
http://forums.alliedmods.net/showthread.php?t=74905
Found in 10 Seconds with the Forums' search function. D;
Note that the above is not the whole secret. Conduct your own research on the plugin linked to above. ;)