When I purchased 2018 there were 3 links in the E-mail, x32, x64 and symbols zip file. You could check your E-mail and see if the link to downloading the zip file is there. If the link to the symbols is not in the E-mail, you will probably have to contact the supplier.
If the link is there, download and extract the zip file, with TC 2018 closed, navigate in windows explorer to the extracted folder and to 'setup.exe'. The symbols will install in 'program files x86', (over 11,000 files) and TC 2018 linked to them. The files are accessed in TC2018 library in a folder called 'Symbols'. Or, you can just link to the symbols you already have following John's advice.