True, but it is a full functioning trial version. I didn't read the PDF...
It is a language translation tool.
A quick look at the online documentation did not find any installation instructions for linux. I assume that that there will be a read me file once you expand the downloaded file.
Only applications that have been preconfigured to run will be available in the software center. Programs that you download like wordfast that are in a tar.gz format are typically source files that have to be compiled for your system.
To extract the file in its current directory:
tar -zxvf Wordfast_Pro_3.4.2_32-bit.tar.gz
Hopefully there will be a directory created where you can then look for a readme or similar file that includes the instructions.