you need to use a sinus calculation.
this shoulnt be this awfulf long ....

this may be wrong but, this should it look - just have a look at the manual to fix that:

function missiles()
{
ent_create("missile.mdl", vector(plAIR.x + 50 * sin(plAIR.pan),plAIR.y + 50 * sin(plAIR.pan,0), rocket);
ent_create("missile.mdl", vector(plAIR.x - 50 * sin(plAIR.pan,plAIR.y + 50 * sin(plAIR.pan,0), rocket);
wait(1);
}


if you spoke of this, im sorry but i dont know how to do this but using vertex coords(vec_for_vertex...)