I've unchecked option

"Always show UI"

and don't see TwainImporter configuration dialog after that. How to enable it again?

You can enable this setting back in INI file. It is located at
for Windows 10:
for Windows XP:
C:\Documents and Settings<username>\Application Data\ScanWorks\TwainImporter\TwainImporter.ini
Please change <username> to your username that you are logged. E.g . for administrator this will be for Windows 10: C:\Users\Administrator\AppData\Roaming\ScanWorks\TwainImporter\TwainImporter.ini
In ini file change this line
It will enable showing TwainImporter configuration dialog again.

I've installed TwainImporter but don't see it in "Windows Start" menu. How to start using TwainImporter?

According to TWAIN specification TWAIN drivers are installed into %windows%\twain_32 folder so installer doesn't ask users where driver should be installed. After installation TwainImporter is available in TWAIN "Select Source" dialog so you can select it as regular scanner driver.

Can I purchase TwainImporter with Purchase Order?

Sure, you can purchase with PO. Please go to BuyNow web page and Select a payment method = "Purchase order" as it is shown on screenshot below.