illegal iso error

Hello,

I am surely missing something obvious, but i get the ‘illegal iso” message when installing openlara in dosbox with my legit purchased Steam version. Placing the openlara.exe in the TR Remastered steam folder gets me a directory error message. 
Been looking around for solutions, but i seem to be the only one simply unable to install openlara on both ends ><. 
mea culpa & thx in advance!