The bolt rib guide has to be lifted up slightly and pushed to the side (left side IIRC, with the bolt facing away from you). To put the bolt back into the rifle (thinking of my Krag carbine at the moment), you align the bottom locking lug with the bolt track in the receiver and as you push forward, you can move the bolt rib back into line. I think you may have to rotate the bolt slightly as well during the process but just very slightly.
Someone who has used their Krag recently can likely provide a better description.
I hope this helps.