How to convert this XML to CSV using PowerShell 2.0?

It may be a little messy but it gets you exactly what you're looking for.

[xml]$convertMe = Get-Content $jsonTemp
[Array]$MeConverted = $convertMe.GetElementsByTagName('item')
$Collection = @()
ForEach($Record in $MeConverted){
    $Output = new-object psobject
    $Record.selectnodes("*")|%{Add-Member -InputObject $Output -MemberType NoteProperty -Name $_.Name -Value $_.'#text'}
    If($Collection){
        $T2Keys = $Collection|gm|?{$_.MemberType -match "Property"}|Select -ExpandProperty Name
        $T1Keys = $Output|gm|?{$_.MemberType -match "Property"}|Select -ExpandProperty Name
        $KeysToAdd = $T2Keys|?{$T1Keys -notcontains $_}
        $KeysToAdd|%{$Collection|Add-Member $_ ""}
    }
    $Collection += $Output
}
$Collection | Export-CSV file.csv -notype

Edit: Frode's looks a lot cleaner in my opinion, but mine does have the advantage of not having to know all of the child nodes' names.

Edit2: Fixed glaring flaws, obviously there will be more than one item and I totally didn't account for that. That just made my code a lot bigger because I need to account for additional potential properties. Now for each item it checks for additional properties that weren't in the collection before and adds them before adding that record to the collection.

Edit3: Updated the Add-Member command for backwards compatibility. I wasn't aware that -NotePropertyName/Value are v3+ until now.

I threw together a little PowerShell function that does this. It assumes a lot about your XML structure, but it worked with the content you posted. You'll just have to pipe it to an Out-File to get your CSV.

Function ConvertFrom-XMLtoCSV {
    [CmdletBinding()]
    <#
    .Synopsis
       Convert a uniform XML file to CSV with element names as headers
    .DESCRIPTION
       Takes a uniformed XML tree and converts it to CSV based on the XPath given.

       For example, assume a structure like this:
       <root>
           <item>
               <element1>Content1</element1>
               <element2>Content2</element2>
           </item>
           <item>
               <element1>Content1</element1>
               <element2>Content2</element2>
           </item>
       <root>

    .PARAMETER Path
        The path to the XML File

    .PARAMETER XPath
        The XPath query to the items that should be converted

    .EXAMPLE
       ConvertFrom-XMLtoCSV -Path .\file.xml -XPath "//item" 
    #>
    Param (
        [Parameter(Mandatory=$true,ValueFromPipelineByPropertyName=$true,Position=0)][String] $Path,
        [Parameter(Mandatory=$true,ValueFromPipelineByPropertyName=$true,Position=1)][String] $XPath
    )
    Begin {
        if (Test-Path $Path) {
            [XML] $XML = Get-Content $Path -Raw
        } else {
            Throw [System.IO.FileNotFoundException] "XML file was not found at the given path"
        }
        $NodeCount = $XML.SelectNodes($XPath).Count
        $FileHeaders = [System.String]::Join(",",$($XML.SelectNodes("$XPath[1]/node()") | ForEach-Object { $_.ToString()}))
        $Content = @()
    }
    Process {
        $Content += $FileHeaders
        For ($i = 1; $i -le $NodeCount; $i++) { 
            $Content += [System.String]::Join(",",$($xml.SelectNodes("$XPath[$i]/node()") | ForEach-Object {$_."#text"}))
        }

        return $Content
    }
}