How can I apply an efix or ifix?

You don't apply interim fixes (ifix) or emergency fixes (efix) with installp - instead you do it with the Efix Manager. IBM provides these fixes in a compressed epkg format (suffix: .epkg.Z). And that's how it's been applied:

# emgr -e <EFIX>.epkg.Z

You get a list of all installed fixes with

# emgr -l
ID  STATE LABEL    INSTALL TIME      UPDATED BY ABSTRACT
=== ===== ======== ================= ========== ================
1    S    IZ79677  09/16/10 16:09:52            iFix for IZ79677

The Label from the table above is needed when you ever want to remove an efix from the system:

# emgr -r -L <LABEL>

With a TL or SP upgrade installp will automatically remove an interim fix only if the service pack already contains it. If not the upgrade will fail and you have to remove it with the efix manager before upgrading.