Try this worked for me. Once in fastboot remove fastboot cable then plug in cable that came with kindle. Then run command you need to run.
Sent From My Rooted Kindle Hd 7"
Edit: If that dont work remove all sdk and files you have put in the sdk platform-tools folder and java you have installed and follow this. Sorry if the sounds like its for a noob but it works.
First things first lets get your sdk and drivers setup correctly.
1. Download the sdk package from here
http://developer.android.com/sdk/index.html#download
2. Open the file you downloaded and take the sdk folder and the sdk manager file and copy it to you hard drive. C:/sdk C Being the hard drive letter.
3. Now you need to open up sdk manager and go to tools menu and select manager addon sites. Now select user defined sites tab and select new and enter this site
http://kindle-sdk.s3.amazonaws.com/addon.xml and click ok.
4. You need to click the following boxes to install the packages you need. TOOLS, ANDROID 4.0.3 (api 15), and EXTRAS. Then click install packages to install them.
5. Now you need to install JDK from here
http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads-1880260.html
6. Now you need to install your basic java from here
http://www.java.com/en/download/index.jsp
7. Now you need to install kindle fire hd adb drivers from here
http://xdaforums.com/showthread.php?t=1890413
8. Now shut down your kindle and connect your kindle to your pc and let it boot all the way and unlock the lock screen.
You should now be able to use your fastboot cable by shutting down and plugging it in.