I just bought the 757, and when I tried to install it I got the messages "error writing to file ace_757.exe" I have tried 3 times and it still gives me the message every time. Please Help!

When installing, did you turn off/disable your Anti Virus program, and run the installer using "Run as administrator" (right click on the installer and select it from the list that pops up)?
After you finish installing using the above method, and hopefully the ace_757.exe gets installed, you might need to add FSX/FSX-SE/P3D to the exclusions list in your AV program, as it might believe that the ace_757.exe is virus or trojan, when it is not.

I have tried with all of my anti-virus software off including windows defender and I still am getting that message at the beginning of the install.

I was able to completely install the CS757, but whenever I try to open the ACE app, it states that it is looking for the ace_757.exe.

I'm pretty sure that your AV, once you turned it back on, removed the ace_757.exe file, which is why I said you might need to add FSX/FSX-SE/P3D to the exclusions list in your AV program, as it might believe that the ace_757.exe is virus or trojan, when it is not.

You can check and see if it is in the AV virus vault/chest, and then restore it to it's installed location, but once you do that, you will need to at least add the location folder for ace_757.exe (FSX/P3D\Captain_Sim\757-III\ace) to the exclusion list of your AV program, or it will remove it again.