1) Got to virtuemart.com and download the latest version
2) Once you have the Virtuemart package – Unzip it .
3)Go to your Joomla Admin page and click Components. In Joomla! 1.5 click Extensions # Install/Uninstall in the Top Menu.
4) You can use the first option –> Upload Package File and select the file ‘com_virtuemart_1.1.x.zip’. Now click ‘Upload and Install’.
5) This should work, if this gives an error message or you were not able to install it – Dont Worry, Joomla provides you other options.
6) Now, Upload [Cut/Copy] your ‘com_virtuemart_1.1.x.zip’ into /media or /tmp folders on your Joomla! site.
7) Goto the third option on your Extension Manager –> Install from URL, give the url to the file you have uploaded.
8) Hit the INSTALL button.
9) VirtueMart should be installed now and you’ll see the Welcome Message Screen with some options.
If this message appears on your screen, you have successfully installed Virtuemart