I'll bet a thousand dollars you are running a vista machine.
You get "Error missing main.cs" because when you download the client vista likes to filter out all
of the important files.
If you have another computer, download it on there and then install it, move the whole blockland folder onto
a jump drive (flash drive) then take it to your computer and throw it in C:\Program Files.
You'll have to run Blockland from inside the folder because shortcuts aren't linked properly in vista either.