I've created my project on Visual Studio 2008, as well RDLC files on it. But now, when I open the solution on Visual Studio 2010 and want to open RDLC file, it's showing me a warning.

That's a little funny. The report was created on VS2008 and VS2010 is asking to convert to 2008 format. Perhaps there was a problem on my VS2008 installation that created RDLC files using some ancient format (2005??!)

The problem is, when you confirm with Ok button, do some design ajustments and run the app, it throws an error on 'Main report':

ex.InnerException
{"The definition of the report 'Main Report' is invalid."}
    [Microsoft.Reporting.DefinitionInvalidException]: {"The definition of the report 'Main Report' is invalid."}
    Data: {System.Collections.ListDictionaryInternal}
    HelpLink: null
    InnerException: {"The report definition is not valid.  Details: The report definition has an invalid target namespace 'http://schemas.microsoft.com/sqlserver/reporting/2008/01/reportdefinition' which cannot be upgraded."}
    Message: "The definition of the report 'Main Report' is invalid."
    Source: "Microsoft.ReportViewer.Common"
    StackTrace: "   at Microsoft.Reporting.ReportCompiler.CompileReport(CatalogItemContext context, Byte[] reportDefinition, Boolean generateExpressionHostWithRefusedPermissions, ReportSnapshotBase& snapshot)\r\n   at Microsoft.Reporting.StandalonePreviewStore.StoredReport.CompileReport()\r\n   at Microsoft.Reporting.StandalonePreviewStore.StoredReport.get_Snapshot()\r\n   at Microsoft.Reporting.StandalonePreviewStore.GetCompiledReport(CatalogItemContext context, Boolean rebuild, ReportSnapshotBase& snapshot)\r\n   at Microsoft.Reporting.LocalService.GetCompiledReport(CatalogItemContext itemContext, Boolean rebuild, ReportSnapshotBase& snapshot)\r\n   at Microsoft.Reporting.LocalService.CompileReport(CatalogItemContext itemContext, Boolean rebuild)\r\n   at Microsoft.Reporting.WinForms.LocalReport.CompileReport()"
    TargetSite: {Microsoft.ReportingServices.ReportProcessing.PublishingResult CompileReport(Microsoft.ReportingServices.Diagnostics.CatalogItemContext, Byte[], Boolean, Microsoft.ReportingServices.Library.ReportSnapshotBase ByRef)}