You'll need to use either AL11.AL_BYTE_OFFSET, AL11.AL_SAMPLE_OFFSET, or AL11.AL_SEC_OFFSET.
You'll probably want to use AL11.AL_SEC_OFFSET, but beware that it rounds the seconds as best it can to the bytes, so bytes and samples will be more accurate.
IntBuffer secondsOffset = BufferUtils.createIntBuffer(1).put(5); //Play at 5 seconds in
It's kind of dumb that they didn't add this until version 1.1; I'm not sure if this was on purpose or on accident, unless there is another way to do this in version 1.0. This code works for me though in LWJGL.