Whenever the "Can't open input file *.csv" message box is displayed by the CSV2FXT script, you first need to check if the .scv file is indeed present in your metatrader's data folder under MQL4/Files.
If all is OK, most likely the symbol of the chart you are running the CSV2FXT script on and your CSV file are named differently.
In that case, you need to do one of two things: Either rename the .csv file to "{chart symbol}.csv" or enter the full .csv file name into the "CSV filename" input of the CSV2FXT script so it will know which file to use for input.
Note: In the default configuration (where the CSV filename is left blank), the CSV file name should always match the symbol name on the chart as shown below:
For example, if the symbol name is EURUSD - the CSV file should be named EURUSD.CSV and that file should be located in the MQL4/Files sub-folder of MT4's data folder.
The data folder is accessed from MT4's File menu via "Open Data Folder"
IMPORTANT: Please make sure that you are not adding an additional ".csv" suffix in the file name. The file needs to be named {symbol}.csv not {symbol}.csv.csv