You're using curvy-braces when you should be using parentheses.
A where statement is kept inside a scriptblock, which is defined using curvy baces { }. To isolate/wrap you tests, you should use parentheses ().
I would also suggest trying to do the filtering on the remote computer. Try:
Invoke-Command -computername SERVERNAME {
Get-ChildItem -path E:\dfsroots\datastore2\public |
Where-Object { ($_.extension -eq "xls" -or $_.extension -eq "xlk") -and $_.creationtime -ge "06/01/2014" }
}