hi
BTPro cliente CAN'T be in a totally diferent folder, because would be impossible to include all necessary files (and some are from user side). So, and as the ReadMe.txt file included says:
Installation instructions:
- ** It's recommended to make a full backup of the OpenTTD instalation folder **
- Extract "openttd.exe" and "lang" folder to OpenTTD instalation folder (usually in C:\Program files\OpenTTD). Overwrite them, if necessary.
- Run the game, enter openttd.btpro.nl website info (Username and Password) in OpenTTD Intro menu.
- Click in the "BTPro Login tool" ingame to login into your account automatically (available too under own nickname in Client List or pressing N key)
Any doubt, feel free to ask and I hope this one helps