How can I install flashable zips in Android-x86
This is caused by the install script incorrectly detecting your platform as ARM.

In order to fix this issue, which is specific to the Xposed ZIP file, you must hack edit flash-script.sh.

Search for:

if [ -z $XVALID ]; then
  echo "! Please download the correct package"
  echo "! for your platform/ROM!"
  exit 1
fi


Comment out the exit command, by adding a # before it.
At least in the current version of Xposed (v86), this is line 162 of the flash-script.sh file.

The resulting block will be:

if [ -z $XVALID ]; then
  echo "! Please download the correct package"
  echo "! for your platform/ROM!"
  #exit 1
fi



Source: How to Install Xposed on MEmu Lollipop Beta


[Edit] Since you are having a great deal of trouble, I'm going to provide detailed fool-proof steps for successfully installing Xposed on Android-x86:


Download Xposed to your computer
Extract the contents of the compressed file somewhere
From your file manager (Finder | Windows Explorer | Nemo | Some other), open the directory where the file contents have been extracted to and browse to: META-INF \ com \ google \ android
Select the entire contents of this directory and Control + C
Go back to the directory containing the resulting files (same as Step 3) and then Control + V
Among the files you have just copied is flash-script.sh. Now you must edit this file with a text editor of your choice, according to the previous part of this answer.
Save and close the file. Make sure it is actually saved.
Make a new .zip file with the updated contents, including of course the flash-script.sh file which you have edited.
Send this new (make sure!) zip file to your Android-x86 environment with a method of your choice. ADB, e-mail to yourself, some anonymous temporary file upload service, etc.
From Android-x86, extract the contents of this new zip file.
Launch Terminal Emulator.
Type in su and hit Enter
If you are asked for Super User privilege, confirm (i.e. hit "Allow")
Type sh "flash-script.sh" and hit Enter
Reboot your phone and launch the Xposed APK to confirm whether it's been successfully installed.