You have to tell us about the mod you're downloading, i.e. which mod are you talkng about? With the name, links, links to the mirrors and so on. Sometimes the .exe is inside the .rar archive.
But very rarely(and I hope it's never the case with Pocketplane mods) the exe not included, in which case the user is supposed to download the latest Weidu version and rename it into Setup-TheNameOfTheModsFolderForTheModYouDownloaded.exe, i.e. if the folder's name is GhostMod, then it's Setup-GhostMod.exe. Then you move the folder and the renamed Weidu.exe file into your BG2 directory and run the renamed Weidu.exe file(which must not be inside the mod's folder, but in the main BG2 folder).