I just got a couple of days ago the 757-300 expansion pack, installed it, got a livery installed, and did some testing with in for GSX alignments and everything was great. I've had the 75-2 base since it was released. Today I went to do my 1st flight in the 300 and I get redirected to the CS website saying that there is an update to 1.5 available. I downloaded it fine, went to install it, I get to the page that says the base pack is required which is obviously installed also and click next. The download instructions page for the update opens again in a new tab on my browser and the installer says the update was not installed but gives no reason as to why.

Something is clearly not right here. I have the 75-2 installed, I have the 75-3 installed, I make sure everything virus/firewall/defender related is off. I load the 75-3 in the sim and I get the update website opened so the installed version is still 1.4 ... is the installer not seeing that the aircraft is installed? Is there an issue with the installer? Is the aircraft actually being updated but the installer says it is not? Given that I've spent close to $100 for this aircraft (75-2 base + 75-3 exp) I would expect to be able to get updates on it without having to troubleshoot a problem as simple as opening an installer.

I'm not sure because you don't specifically say this, but do you have the 757-200PW Base Pack v1.5 installed?

If it isn't installed, then that could be why the 757-300 v1.5 won't install (when I tested installing the 757-300 v1.5 with the 757-200PW v1.404 (with Hotfix 1.412) it wouldn't install).