The easy and consistent way is to use your Ubuntu package manager, Synaptic or Ubuntu Software Center. The Ubuntu repositories contain TeX Live bundles, but their names often don't match a specific package. So, use the search feature to search the package descriptions for the package name. Click to choose and install.
You could install (extract, copy) the package into your user home directory, to ~/texmf. This is an option if Ubuntu really doesn't provide it or if you would like to use a newer version. In that case, download it from http://ctan.org/pkg/packagename. There are often installation instructions available. On this site, you could additionally check questions regarding installation of packages in general.
If adding is what you just need then this answer might work for you. If you want to install a new dictionary, follow the link in the comment. With your terminal, cd to the directory of your TeXMaker dictionary. If you don't know where, in TeXMaker click
Options > Configure TeXMaker > Editor
You can see the directory of the dictionary in the option Spelling dictionary. In Ubuntu, once you are in the directory, type
sudo gedit en_GB.dic
and put the word you want to add in there.
In Windows, just go to the directory and double click on the file or open with Notepad. Then write the word you want to add.
You might also want to save the customized file in another folder and have your Spelling dictionary option point to the custom file.
Best Answer
Under User --> User Commands you can add up to five custom commands.