For now don't use the itch manager until the developer adapts it (he recently commented that he contacted itch.io to increase the limit of 1GB per link). Manually download each file, for example from the email you received for the purchase you should have a download link. Download the 4 files (.exe .001-.003). If they are all in the same folder, run the .exe and wait.
If you want to use the Itch.io app you can cheat the program and install from the manager only the SlaveBar v1.3 M.exe (this way itch will create a specific folder). Move everything you unzipped earlier to this folder and you should be able to run the game from the itch.io app by selecting the correct exe.
The folder structure for the manager should look something like this:
-.itch*
- New folder**
- SlaveBar v1.3 M***
* Folder auto created by itch.io app
** Here pull the exe and the rest of the parts
*** Here are the game data
I hope this helped you in some way and sorry that English is not my first language.
