After creating a custom drawing mode and customizing the Parts Database Manager to add the field for the symbol name, the field does not have an ellipsis [...] button to browse for symbols like the standard symbol fields do.
It may not be necessary to perform all of the following.
In Mode Manager make sure the name of the custom drawing mode ends with the word "Mode".
On Setup > System Paths, select the Edit button to edit the parts database path and go to the field mapping step. Make sure the field that is supposed to contain the symbol names for the custom mode has "Symbol Data" specified in the Category column and a "user" field specified in the Internal Name column. Note the Display Name value.
In Mode Manager, make sure the field name in the Parts Database Field column matches the Display Name noted in the previous step. If not, be sure to click Apply after correcting it.
The symbol field for the custom mode should be on the Symbol Data tab of Parts Database Manager. If it already is, select the Customize Layout button and find the field in the tree. Right-click the field and select Size Constraints > Reset to default.
Restart the software.