Open the .dmg

You’ll find the .mpkg. Right*click and “Show Package Contents

Open Contents Dir

Open Packages Dir

Install each of the 4 packages in this order:
1. OSXFUSECore.pkg,
2. OSXFUSEMacFUSE.pkg,
3. MacFUSE.pkg,
4. TrueCrypt.pkg

Taken from the source listed below.  They also have other ways of accomplishing the same thing, but I found the way listed above to be the easiest and least technical

Source: https://lazymind.me/2014/10/install-truecrypt-on-mac-osx-yosemite-10-10/