Currently Personal Capital does not allow for manually entering transactions into your linked or manual accounts. If you are missing any transactions from your linked accounts, you can manually refresh your data to pull in any missing data by following the steps here: How Do I Force Data To Be Refreshed For A Specific Account
As for exporting transactions, this is possible from the Transactions page (Overview->Transactions). From there click the CSV icon to export your transactions:
This will download your transactions in a CSV file.