How to add a mod to you current world in Space Engineers

Log into your game panel and click 'stop' on your service.

Step 2:
Select File Manager from the icons
Step 3:
Once in File Manager navigate to /settings/saves/Your Loaded Worlds Name

Step 4:
Find the file called "Sandbox.sbc" and edit it using the icon
We suggest setting this as a favourite by clicking the star icon
This will add it to the Configuration Files for easier access in the future

Step 5:
Navigate through the file until you find the line:
<Mods>

Step 6:
Find the mod ID you wish to use
You can get the ID from the URL of the mod:

http://steamcommunity.com/sharedfiles/filedetails/?id=297110720 <- this number

Step 7:
Add the ModItem ID like so:

<Mods>
   <ModItem>
     <Name>xxxxx.sbm</Name>
     <PublishedFileId>xxxxx</PublishedFileId>
   </ModItem>
</Mods>

For example:

<Mods>
   <ModItem>
     <Name>297110720.sbm</Name>
     <PublishedFileId>297110720</PublishedFileId>
   </ModItem>
</Mods>

Step 7.1:
If you are adding more mods, it is done like the following:

<Mods>
   <ModItem>
     <Name>xxxxx.sbm</Name>
     <PublishedFileId>xxxxx</PublishedFileId>
   </ModItem>
   <ModItem>
     <Name>xxxxx.sbm</Name>
     <PublishedFileId>xxxxx</PublishedFileId>
     </ModItem>
</Mods>

Step 8:
Restart the server and the mod will load and install

Note:
If you are watching the 'Web Console' you will see the following line:

2014-08-1413:32:49.796-Thread:   1->        Up to date mod:  id =xxxxxxxxx

Credit: http://forums.keenswh.com/post/dedicated-server-mods-files-and-configuration-7037734?pid=1283915761#post1283915761