You can't use IPSM for Optipoints or OS15 (CP phones only), so you'll have to use DLS. Which is a shame because IPSM is way, way simpler.
DLS syncs to the 4K using the config under "Element Manager", it logs in to the Assistant. You can look in the Element Manager and make sure it's periodically syncing, password is correct, etc. It's no use if it hasn't synced for a year. If the node code (I think) isn't populated in the element manager when you add the 4K, the 4K phones won't appear in the DB, so if you make a sync and nothing appears, check that.
To get the physical phones in the DB, you have to either scan them from DLS, or have the phones register on the DLS by giving them the DLS IP over DHCP. If you have issued the wrong DLS address, you can't then overwrite that by scanning them and telling DLS to give the phones the right one, you'll need to correct it in the DHCP server.
You can get duplicate entries in DLS by resyncing but specifying a different node number in the element manager, specifying a different node number on the phone etc (by that I mean via terminal number, sometimes people enter the full E164 in DLS but a node code+number on the phone terminal number), you have to pay attention. With a small site and one 4K it can be easier to just delete everything and start again, but even then the DLS might have configured some phones with node code+extn, and some with E.164 in the terminal number, so when you scan them again it's still a mess. HFA doesn't use the terminal number for registering on the 4K (like SIP does), HFA uses the system config page for that where you give it the subscriber number, 4K IP etc, but it will use terminal number to create host name and DLS uses it to populate the DB.
If you've never used it, you won't have these problems.