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Is this the default activities or and add-on activity?
If its default then for some reason windows 7 wont install MSTS properly to the folder MSTS suggests. You have to create another folder to install to. Add on routes can be a different story though. You can find full details here: http://ftp.uktrainsim.com/viewtopic.php?f=273&t=115842&sid=fb1a4552a4aab88ae28d08912c3f0f73
You can find full details here:
have you tried dragging tha apk file over the top of the activity unpackager icon in the UTILS folder in in program files.
It should be C:\My Computer\Program files\Microsoft Games\Train Simulator\UTILS
i'm running xp an that's all i have to do to unpackage an activity i usually download from uk trainsim.