What he gave you was instructions using the Ribbon. I will translate:

1.
Ctrl-A = Select all text in document
Review = click on Review Tab
Language = click on language button
Set Proofing Language = Click on this choice
Uncheck 'do not check spelling or grammar' = make sure this checkbox is clear

OK your way back to your document
2. Word Options = Click on the Office Button and select Word Options
Proofing = Click on the Proofing side tab
Recheck document = run spell check again (I think you could just do this by pressing the F7 key).

This will turn spell checking back on.

If you can, I would recommend setting up a character style that is the same as your body text but has 'do not check spelling or grammar' checked. Then apply that style to the technical words you use and press the F7 key.
See Understanding Styles in Microsoft Word