DreamHost is setup to "automagically" display a file instead of an index of folders/files if you have a file with a certain name in your directory. Any of the following file names will work "out of the box" without you having to do anything:
index.html index.shtml index.htm Index.html Index.htm Index.shtml default.htm Default.htm default.html Default.html default.shtml Default.shtml page1.html index.pl index.cgi index.php index.php3 index.phtml home.htm home.html home.shtml index.wml
That said, there are a couple of was to approach your situation. First, you could rename your "main.html" file to one of the above listed ".html" or ".htm" filenames, and it will display when someone browses to that directory.
If you really want to use "main.html" then you can use an .htaccess file to make "main.html" be displayed via the DirectoryIndex statement.
For more information about doing this, see the DH Wiki articles:
The short version, if you are more interested in "just doing it" rather than learning about it is:
Create a "plain text" file named ".htaccess" containing the single line:
and upload it to the directory containing "main.html". This will cause "main.html" to be displayed whenever anyone visits that directory and does not specify a filename or other more specific url.