If the downloaded file you got is a zip file, you have gotten the wrong file for a Mac -
that is a windows download.
Go to
http://www.openoffice.org/download/ and make sure the dropdown on the left shows the correct information about your operating system and version. You state you are using OSX 10.11.6 so you need
OSX (version >= 10.7)DMG. If necessary, select the language you need. Click on Download full installation - you will be transferred to Sourceforge. Once there click on nothing, your download will start automatically.
Prior to downloading, please carefully read the
Mac installation instructions and also Q03/A03 and Q05/A05 in the
[Tutorial] Mac FAQ