Unable to execute: Add-PSSnapin Microsoft.SharePoint.PowerShell

Before you can do anything with PowerShell ISE, you need to make sure that (1) You are running PowerShell ISE as an Administrator and (2) that the execution policy is set in such a way that you can execute scripts.

Open SharePoint Management Shell and run

Get-ExecutionPolicy

If you get RemoteSigned, Unrestricted or Bypass you can safely continue. RemoteSigned means that you can't run scripts that are downloaded from the another server or the internet unless it is signed. If you download files to run in PowerShell ISE, set the execution policy to unrestricted, like this:

Set-ExecutionPolicy Unrestricted

If all works as expected, run PowerShell ISE as an Administrator. Then try to load the SharePoint.PowerShell with the following:

if ((Get-PSSnapin "Microsoft.SharePoint.PowerShell" -ErrorAction SilentlyContinue) -eq $null) 
{
    Add-PSSnapin "Microsoft.SharePoint.PowerShell"
}

When that has loaded, try another command, such as

Get-SPManagedAccount

If you receive no error messages, then you have SharePoint.PowerShell loaded. If not, run

Add-PSSnapin "Microsoft.SharePoint.PowerShell"

and read the error messages. Copy the first line of the error received and try to search for it. Still in trouble? Update your answer with the error message, and I'll try to help if you notify me in a comment.