Here's is just a snippet from my Monarch Class Library.
Basically, you want to get the summary count, then loop through the summaries, set it as the current summary and then export each one.
If SummaryCount = 0 Then
blSuccess = False
sProblem = "No Summaries found in: " & sModelFile
Err.Raise vbObject + NoSummaries, "Monarch", sProblem
Set mstrsFiles = New Collection
For SummaryIdx = 0 To SummaryCount - 1
SummaryName = UCase(.GetSummaryNameAt(SummaryIdx))
blSuccess = .JetExportSummary(sExportFile, SummaryName, iAppendFlag)
If blSuccess = False Then
sProblem = "Error Exporting Summary: " & SummaryName
Err.Raise vbObject + SummaryErr, "Monarch", sProblem
mstrsFiles.Add Key:=SummaryName, Item:=msSummary