You have to register Think function and this function should look like that:
PHP Code:
new Float:FlVecOrigin[3];
entity_get_vector(entity_index, EV_VEC_origin, FlVecOrigin);
// THIS ARE JUST EXAMPLES
FlVecOrigin[0]+= Number_X; // X coordinate gets bigger with Number_X units
FlVecOrigin[1]-= Number_Y; // Y coordinate gets lower with Number_Y units
FlVecOrigin[2]+= Number_Z; // Z coordinate gets bigger with Number_Z units
// However you said "move around" and if I understand it correctly, you don't need changing Z axis
entity_set_vector(entity_index, EV_VEC_origin, FlVecOrigin);