I have installed SilverStripe - 2.4.1 and am attempting to get Blog [v0.4.0] working. I have made a sub dir called /blog and unpacked the files from blog-v0.4.0.tar.gz into it.

\blog
- \blog-0.4.0
- - \(all other files/dir's from packed file)

I do the index.php/dev/build?flush=1 and get the following:
Creating database tables
# PageComment
# SiteConfig
# SiteTree
# Email_BounceRecord
# QueuedEmail
# File
# Group
# LoginAttempt
# Member
# MemberPassword
# Permission
# PermissionRole
# PermissionRoleCode
# Widget
# WidgetArea
# Page
# ErrorPage
# Index ErrorPage_versions.RecordID_Version: changed to unique (RecordID, Version) (from unique (RecordID,Version))
# RedirectorPage
# Index RedirectorPage_versions.RecordID_Version: changed to unique (RecordID, Version) (from unique (RecordID,Version))
# VirtualPage
# Index VirtualPage_versions.RecordID_Version: changed to unique (RecordID, Version) (from unique (RecordID,Version))
# Image
# Folder
# Image_Cached

Creating database records
# PageComment
# SiteConfig
# SiteTree
# Email_BounceRecord
# QueuedEmail
# File
# Group
# LoginAttempt
# Member
# MemberPassword
# Permission
# PermissionRole
# PermissionRoleCode
# Widget
# WidgetArea
# Page
# ErrorPage
# RedirectorPage
# VirtualPage
# Image
# Folder
# Image_Cached

Database build completed!

I can see the interface in the admin panel but when i click on the blog link that was added to my sit all i see is a directory listing for the /blog/ dir

Index of /blog

* Parent Directory
* blog-v0.4.0/

what have I done wrong?

I have the same problem!

I have extracted the files from the archive, copied the files into a blog folder in my site's root.
I get the directory index, listing the files and folders.

Index of /blog

[ ]   ChangeLog    17-Dec-2008 04:04    1.9K   
[ ]   _config.php    30-Nov-2009 11:04    85    
[DIR]   code/    28-May-2010 05:16    -    
[DIR]   css/    28-May-2010 05:16    -    
[DIR]   images/    28-May-2010 05:16    -    
[DIR]   javascript/    28-May-2010 05:16    -    
[DIR]   lang/    28-May-2010 05:16    -    
[DIR]   templates/    28-May-2010 05:16    -    
[DIR]   tests/    28-May-2010 05:16    -    
[DIR]   thirdparty/    28-May-2010 05:16    -    

Would it be anything to do with the fact that I'm using a custom theme for my site?

Thanks,
Tim