Let me see if I can help.
So I'm assuming you've downloaded the file called setup-owncloud.php to your computer.
Step 2 is simply using an FTP client to upload it to your DreamHost account. If your site is called my-site.com, you can put the file in the my-site.com directory.
In your browser, go to my-site.com/setup-owncloud.php and follow the instructions for setting it up (creating admin user, password, etc.). The default settings are fine.
The recommended settings are not strictly necessary, but will speed up and secure ownCloud.
Now we'll configure DreamObjects with OwnCloud - http://wiki.dreamhost.com/How_to_Use_ownCloud_with_DreamObjects
Make sure you've enabled DreamObjects and created a bucket in the DreamHost Panel - https://panel.dreamhost.com/index.cgi?tree=cloud.objects& Keep this page open in your browser, you'll need the info from here soon.
Once you've finished the ownCloud setup steps, login and go to the apps section by clicking the "+ Apps" in the bottom-left corner. Find "External storage support" and enable it.
Now go to the Admin section by clicking your username in the top-right then clicking "Admin".
In the "External Storage" section, enter the following info:
Folder Name: DreamObjects
External Storage: Amazon S3
Access Key: Your Access Key from the DreamObjects page in the panel ( http://wiki.dreamhost.com/DreamObjects_End_User_Guide#Keys )
Secret Key: Your Secret Key from the DreamObjects page in the panel
Bucket: Your DreamObjects bucket name from the DreamObjects page in the panel
Enable SSL: Checked
Enable Path Style: Checked
Applicable Users - Select yourself from the list
Once you've done that, you should see a green light meaning it connected.
Now you can download the desktop and mobile apps from ownCloud and use it similarly to DropBox http://owncloud.org/install/