There are two different, unrelated sln-mode libraries:
Both have/are a file called sln-mode.el.
This is an add-on library for ProjectBufferMode.
This library provides a function to create a project-buffer extracting the information from a SLN file and its associated VCPROJ.
(autoload 'find-sln "sln-mode")
‘find-sln’ is the command to execute to open a ‘sln project’.
Check ProjectBufferMode for more documentation about it.
The command line are different between a 2005 and a 2008 project; by default it will use the 2005 configuration mode; use the prefix argument to switch to the 2008 mode.
sln-mode is currently just a viewer. It doesn’t currently have any support for modifying/writting a SLN file.
Note that it doens’t have to stay that way if people really need this feature. ;-)