In order to export Invoices to MYOB from Briefcase you should ensure that the client name used in Briefcase is setup as a contact in MYOB and also that the name between Briefcase and MYOB is the same.
You should also ensure that you have integrated Briefcase with MYOB, for more information regarding this review the MYOB integration article.
To batch export invoices from Briefcase to your version of MYOB you can go to the 'Invoices' tab. You can then use the filter menu to filter by 'Open' status invoices. You can also sort/group invoices to filter out the ones you do not want to export. Once you're ready, press the 'Click here to export * invoices" link.
If you would prefer to only export out individual invoices, you can do these by viewing the invoice you would like to export, press the action menu and selecting option 'Export Invoice'.
Once this is done you'll need to open your MYOB application and navigate to the File Menu. Select Import Data > Sales > Service Sales in the menu. Once the Import File dialog appears you will need to ensure that the following options are set:
Import File Format: Tab-delimited
First Record is: Data Record
Duplicate Records: Reject Them
Identify Contact By: Co/Last Name
Once you've confirmed the above select the 'Continue' button and locate the exported Invoice file from Briefcase. Once the Import Data dialog appears ensure you select the 'Match All' button and then press 'Import'. MYOB will then present you with the Import Service Sales pop-up which should not have any skipper records - If you notice that there are in fact skipped records you will need to review the import log that MYOB provides to determine the nature of the error. For more information related to MYOB errors please review the article: Exporting errors in MYOB.