It sounds you use a modeless UserForm, for which you need to use AcFocusCtrl in order for the controls on the form to maintain focus.
However, with 64bit AutoCAD of older version (prior to Acad2014), the VBA is still 32-bit and runnas external process synced with AutoCAD, there are quite some reports that modeless userform works not as expected, even with AcFocusCtrl beign used.
So, if possible, try avoid to show userform as modeless.
Anyway, to get AcFocusCtrl, you righ-click UserForm's toolbox and select "Additional controls...", you then should be able to see "AcFocusCtrl" in the "Additional Controls" dialog box, usually the first item. Check the leading check box and click "OK".