Hi Richard,

I have found the way to install AutoCAD LT 2013 on Yosemite by using AutoCAD_LT_2013_English_Mac_OSX_R1.dmg installation image.

If you really cannot wait for the solution from Autodesk, I think you can use my trick:

At first I need to say that you should be very careful using this method in order not to break Yosemite on your Mac.

So here are the steps:

1. Locate SystemVersion.plist file inside /System/Library/CoreServices/ folder on your Mac, backup this file somewhere in safe place, than copy it to your Desktop and open with TextEdit.app. You will see this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>ProductBuildVersion</key>
	<string>14A389</string>
	<key>ProductCopyright</key>
	<string>1983-2014 Apple Inc.</string>
	<key>ProductName</key>
	<string>Mac OS X</string>
	<key>ProductUserVisibleVersion</key>
	<string>10.10</string>
	<key>ProductVersion</key>
	<string>10.10</string>
</dict>
</plist>

2. Using TextEdit.app edit substitute two instances of <string>10.10</string> with <string>10.9.5</string> 

The contents of the SystemVersion.plist should look like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>ProductBuildVersion</key>
	<string>14A389</string>
	<key>ProductCopyright</key>
	<string>1983-2014 Apple Inc.</string>
	<key>ProductName</key>
	<string>Mac OS X</string>
	<key>ProductUserVisibleVersion</key>
	<string>10.9.5</string>
	<key>ProductVersion</key>
	<string>10.9.5</string>
</dict>
</plist>

3. Save SystemVersion.plist file, quit TextEdit.app. Than copy edited SystemVersion.plist file from your Desktop back to  /System/Library/CoreServices/ folder and chose "Replace", than enter your Admin password to confirm replacement.

4. Now start AutoCAD LT 2013 installation from the  AutoCAD_LT_2013_English_Mac_OSX_R1.dmg installation image. The installation process should go fine now. After installation apply SP2 for AutoCAD LT 2013 without starting AutoCAD LT.

5. Now put original SystemVersion.plist file which you backed up in step #1 back to  /System/Library/CoreServices/ folder and chose "Replace", than enter your Admin password to confirm replacement.

6. Restart your Mac. You will see AutoCAD LT 2013 icon in the Dock.

7. Apply SP3 for AutoCAD LT 2013 for Mac.

8. Now you can start AutoCAD LT 2013 for the first time after installation, enter license information (your serial and product key), wait for activation and -  now you can use AutoCAD LT 2013!

Check AutoCAD LT 2013 version, it should be now:

The only one "glitch" I noticed after SP3 installation is that AutoCAD gives a warning about availability of the new update, but if you will download this update - it will be the same SP3 which you installed in step #7, so you can ignore this alert.

Maxim