You should be able to install deb packages by double-clicking on them. Are you getting some error message ?
Alternatively, you can also install a .deb package by opening a terminal and typing:
sudo dpkg -i <package_name>.deb
If all the dependencies are met, it will be installed. If not, the unfulfilled dependencies will be mentioned.
Generally it is easier to install software using apt-get.
Have a look at the following link, which you could find useful:
<a href=”http://www.psychocats.net/ubuntu/installingsoftwareold”>Installing Software in Ubuntu</a>