Content builder ( %localappdata%Staxel.Launcher\gamedata\bin\Staxel.ContentBuilder.exe ) is a tool used to prepare assets to be used by the game, it needs to be run when adding mods to a server.
The Asset Manager is an all-in-one asset management system for Staxel. The default path for the Asset Manager is '<user id>/appdata/local/staxel.launcher/gamedata/bin/staxel.assetmanager.exe.
Start by loading the content system ("File->Load Asset Directory", this will be done automatically if the option is checked in your preferences ("Edit->Preferences")). Everything, except building assets or using the "Create Material Models From Tile Models" tool, requires the content system to be loaded. From there you may begin using tools (under "Tools" in the top menu) or editing assets. To edit assets, open the window for the type of asset you wish to edit (under "View" in the top menu, or press CTRL+Shift+1-9) and have at it.
The Live Content System ("View->Content System Live") shows you exactly what is in Staxel's content directory, and let's you manage (create/delete) files and folders within the content directory safely.
While editing assets, it is sometimes helpful to view the asset's source file. To do this quickly, open an asset window and click "View->Source->File". The source file will be opened in an editor of your choosing, specified in your preferences. If you like, you can provide arguments to the editor in the preferences. If the arguments are left empty the asset's source path will be provided, otherwise, you can use the macro $(fpath) which will be replaced by the asset's source path (e.g. "/f $(fpath)" may be expanded to "/f C:\dirt.tile"). You may also like to open the directory containing the asset's source file via "View->Source->File in Directory".
If you have made changes to any assets, or have added new model files, and are ready to use them in Staxel, it is a good idea to make sure that all your assets are valid and your model files are converted into the format used by Staxel. To do this, click "File->All Assets->Build and Validate Assets". Don't forget to save your assets (in each asset window, "Asset->Save->All Assets") before doing so, however.
[More comprehensive help section coming...]
You do not need to install content mods when wanting to join a content modded server.
Mods go into the Content/mods directory ( %localappdata%\Staxel.Launcher\gamedata\Content\mods )
Every mod should have its down directory there, file should not be left in the mods directory itself.
When making a mod, make sure the use a unique mod prefix (mods.[yourmod]. namespace prefix and not the staxel. prefix) for newly added assets.
Example mod: [[]] (thanks to scornedbythenine)