when you download the installer from them, you'll open that and install nexus. it will install it where it needs to be. then when you start logic, create a software instrument track. when you click the instrument box to load a software instrument, you scroll to reFX then nexus2. it will then prompt you to install the content and you can install expansion and what-not from the system tab. if reFX and nexus2 aren't showing up under your instruments then you haven't installed it. when you go through the installer, there are options to install a bunch of different formats. make sure AU is selected. that is what logic uses. if you've followed the installation all the way through, make sure logic is completely quit and restart logic again. it'll scan for new plugins. it should then be there. if not, you're doing something wrong. but without us being there to see how you're installing it, it's hard to know how..... it's fairly straight forward.
OSX 10.10.1 15" macbook pro retina late 2013, 2.3 GHz i7, 16GB RAM, 1tb PCIe flash drive
Logic Pro X 10.1 and Logic Pro 9.1.8
MId 2011 27" imac 3.4 GHz Inter Core i7, 32GB RAM, 1tb SSD