mirror modifier in 2.5

i can’t get a modifier to work that i liked in 2.49 and feel sure was kept somehow in 2.5, namely the mirror modifier that allows you to construct only half of a symmetrical object, then just mirror what you’ve created.

there’s a mirror menu in 3d view in both object and edit mode. but i can only get it to distort the object i’ve selected rather than to mirror it across its center.

i know i’m missing something and would be very grateful to anyone who can point me in the right direction.

sorry, i think i figured it out. the object i was trying to mirror was a curve rather than a mesh, so mirroring didn’t show up on the add modifier menu.