Many people have been unable to install VirtueMart 1.1.2 since its release. The common error message they were getting was:
There was an error uploading this file to the server. Unable to find install package
Just from this message it's hard to determine what the real problem could be. The main reason is that the component install file (com_virtuemart_1.1.2.j15.zip) is about 2.1 MB in size. Too large, because a maximum upload file size of only 2MB is common for most PHP configurations. If you attempt to upload a larger file, the upload fails, because larger files are not allowed.
How to get around this problem - change the maximum upload file size? There are easier solutions: don't use the web uploader...
...Upload via FTP and install from directory
You can transfer the contents of the file com_virtuemart_1.1.2.zip to the /media directory of your Joomla! 1.0 installation (or /tmp for Joomla! 1.5) using an FTP client. So extract the file locally and upload its contents to a directory in your Joomla! site. Remember the path you uploaded the contents to. Now go to the installer screen, fill in the correct path (where the files reside) and click "Install".
...Install from URL (Joomla! >= 1.5 only)
This is the easiest way to install the VirtueMart component. Go to the Installer Screen, enter the URL "http://virtuemart.net/latestcomponent.php?j=1.5" in the URL text field and click "Install". The component installer file is now downloaded directly to your server, extracted and installed.
Note: this requires a working internet connection from your server (no proxy, URL-fopen enabled).
...Use the Remote Installer Component (Joomla! >= 1.5 only)
If you are using Joomla! 1.5, you can install the "VirtueMart Remote Installer" component, which assists you when it comes to install VirtueMart.
You just need to do some mouse clicks and you're done. Please note that this method also requires a working internet connection from your server (no proxy, URL-fopen enabled or cURL).
If you're Server/PHP configuration doesn't allow that, you need to use the FTP upload method.