Dub / Derelict confusion

[Paul via Digitalmars-d-learn]

I would like to create a simple program using SDL. I've read this 
page http://dblog.aldacron.net/derelict-help/using-derelict/ and 
this one http://code.dlang.org/about and decided that using 'dub' 
would be the sensible option for a beginner so I downloaded the 
dub executable and put it in the same directory as my source file 
'test.d'.

As per the example at the above url, I created a dub.json file in 
the same directory with contents as so:

"dependencies": {
"derelict-sdl2":"~>1.0.0",
"derelict-gl3":"~master",
}

If I have the relevant import statements in test.d, am I right in 
thinking I can use dub to build my program (not just download and 
build the derelict libraries)? The command dub build test.d gives 
me a 'failed to load package' error.

I'm using 32 bit Linux.

Any help appreciated :D

Paul