Hi Niceperson,
If I understood correctly your MT4 broker does not have the symbol ‘USDSEK’. In this case, it is necessary to use another symbol that your MT4 broker does support so you can use it as a proxy for your USDSEK data. For example, you can export USDSEK data and “map it to” the GBPUSD symbol in MT4. Once you have done this, selecting GBPUSD in the strategy tester will actually use the USDSEK data you have exported.
You can find out more about the ‘Maps to’ feature in the manual or you can have a read of this post:
viewtopic.php?f=4&t=435
With EURUSD, it seems you may be coming across a different issue which is that MT4 does not support large test ranges. That is, your FXT data file cannot be more than 4GB. You will need to adjust your exported date range to suit. You can refer to this post for further information on this topic:
viewtopic.php?f=4&t=462
Hope this helps.