Okay.. Well first off, when you first ran your server you should have been prompted on the RunUO console to create an account. That is the Owner (Admin/GM) account for you to log in with, and has full 'power'. Use this account for your 'GM ' character.
Once in game with your Owner character you can create. The XML Spawner has a variety of commands you can use for handling and creating spawners. You can add them one by one by using the command [add xmlspawner and target the ground where you want the spawner. It will place a little gray crystal that is only visible to GM (staff type) characters. Double click that crystal and you will get a gump display where you can now enter in a creature or item to spawn. Click on the 'Properties' button for more functions to provide the spawning details.
Search this forum for XML spawner archive or something along those lines, and you should find a .zip file of a massive collection of html files of documentation on how to really use this spawner to its potential.
If you are after a quick full world spawn, search and download neruns distro. You can then follow some directions with XML spawner to import them to your world. I have mixed feelings about this approach because I find that some areas are okay, and others are just way overkill and much of the areas will need reduced or adjusted to your liking.
You will really need to do some reading and learning on how to use this spawner because there is just way too much to it to explain in one post here. But with the neruns files, once you get to understand xml spawner, you would simply use a command like [xmlimport trammel.map or something of that sort. (that is not accurate, just for the idea of how easy importing a spawn file can be.
Once you have spawners in game and you want/need to locate one, you can use a handy gump/display tool by using a command [xmlfind This will display every spawner crystal in game, and allow you to easily edit them or go to one, etc..
This should get you going at least, being you do have the XML spawner installed to your server. Thats about the more difficult thing, and you said you have that accomplished
Hope that helps a little bit, and gets you going again. Good luck, and feel free to post any troubles you have along the way. Time is limited for me, but I do try to get on here from time to time to help out someone.