@lunita,
Disclaimer: I know nothing about "LiveUSB", a third party offering. I recommend that if you are determined to use it, you contact the author directly for support.
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The OpenBSD Project's only official, supported methods for installing are via booting from install media developed for the particular architecture. The media choices vary. For the two most popular architectures, i386 or amd64, the choices are the same: booting a diskette, booting a CD/DVD, or booting from a network using a PXE capable NIC and and a PXE configured LAN. (PXE booting requirements and configuration are described in the
pxeboot(8) man page.)
If none of these are available on the platform you wish to install onto, the Project recommends either:
A) From a platform that has appropriate media, install onto a bootable, removable external drive, attachable via external channels such as eSATA, USB, or SCSI.
B) From a platform that has appropriate media, swap internal drives, install, swap internal drives.
Both of these recommended methods require at least one additional system of the same architecture which can boot installation media. That system may be virtual (e.g.: qemu) if convenient.
Most common alternative methods -- such as manually creating a bootable USB stick containing just the install kernel and installation filesets -- require the admin have prior knowledge of OpenBSD boot block installation/configuration for the particular architecture.