Well, now it seems that after retrieving the soul essence gem I can't get back to the stairs. There is no portal to go back through. However, you can jump down from there. Still, after doing so i have been unable to get back to the stairway. Last night I spent nearly an hour running around that cursed place trying to find the stairs to no avail. You can't even see it off in the distance anywhere on the map or visually. Anyway, this morning I deleted all of the saved games leading up to level 10 and started over again, much to my displeasure.
Wanderer's Tip:
All roads in the Soul Cairn eventually lead to the same place. The Portal in, or the Watchers/Guardians little throne buildings.
Point is, from any location in the Soul Cairn, following any road can lead you back to the portal.
If all else fails, use the local map in your menu. The Marker still shows which way your character is facing. Follow the lit-up (unhidden) areas of the map (where you've already been), and it will leads you on a path back to where you came from. in the event that you've wandered all over and every place has been discovered, you can get back to the Portal by simply finding your way back to Valerica's prison/hideout. Walk directly away from the prison/hideout (follow the road), and it will lead you back to the Portal steps.
In the event that you are able to find the Portal steps but the Portal itself isn't there, then you have encountered a bug. Simply reload from a previous save and this issue will clear up.
Now, since you've already deleted all of your save files and have started over, use this for future planning/trips into the Soul Cairn.